Guidance

Mergers: how they are investigated

Published 29 April 2025

This guide summarises how the CMA investigates a merger. It should help you if you’re:

• thinking about merging your business with another one
• aware of a merger that you think could affect your business or competition generally

We also have more detailed guidance. For more information about our remit and processes, read our guidance on the CMA’s jurisdiction and procedure (CMA2).

What counts as a merger

A merger is when 2 or more businesses (or parts of businesses) join to become one. This could be through:

• a business buying another business or part of one (an acquisition)
• businesses agreeing to work together on something new (a joint venture)

Our role in merger control

The CMA is the independent, non-ministerial government department responsible for keeping markets open and competitive. One important duty we have is to ensure that UK businesses and consumers are not harmed by mergers that would reduce competition. Those harms include lower quality goods or services, less choice or higher prices.

Tens of thousands of mergers take place every year that we don’t consider – and many more we do not find concerns with. We only intervene if the merger meets certain criteria (see below) and we think it could have a negative effect on consumers, businesses or the UK economy.

When we investigate, we gather evidence from merging businesses and others, such as customers or competitors of the merging businesses. This helps us to decide whether the merger will affect competition. At the end of the investigation, we publish a report explaining our decision.

How we decide to investigate a merger

For the CMA to investigate a merger, it must meet certain criteria, including at least one of the following:

• ‘turnover test’: the business being acquired has a UK turnover of at least £100 million
• ‘share of supply test’: the merged business would supply over 25% of a good or service in the UK (or a region of it); and the merger would increase this share of supply; and one of the businesses must have a UK turnover of at least £10 million
• ‘hybrid test’: one business must supply at least 33% of a good or service in the UK (or a region of it), and must have a UK turnover of more than £350 million, while the other business must have a presence in the UK

How we find out about mergers

Unlike in many other countries, the UK merger regime is voluntary. This means you can choose whether to tell the CMA about your merger or not, and whether to tell us before or after the merger has completed. However, there can be significant benefits to telling us before completion.

There are 2 main ways that you can tell us about your merger:

• submit a briefing paper
• notify us formally

Submit a briefing paper

Merging businesses can submit a 5-page briefing paper to the CMA. This is an option if you are not sure that your merger meets any of the 3 criteria above, and/or you believe it does not raise any competition concerns.

Unlike if you notify us formally, submitting a briefing paper does not necessarily mean that we will investigate your merger. Our mergers intelligence committee will consider the information you provide, and request more if needed. We will then decide whether to investigate formally. It will not be made public that you have contacted us.

A briefing paper is not suitable for mergers that are complex or may need detailed investigation to determine whether the merger could affect competition.

 Email the mergers intelligence committee: mergers.intelligence@cma.gov.uk

Notify us formally

If you notify us about your merger, we will start the first phase of a formal merger investigation (we call this ‘phase 1’) once we’ve decided that we have enough information (pre-notification’). At the end of phase 1, we will publish our decision.

To notify us of a merger, complete a merger case team allocation request form and send it to mergers@cma.gov.uk

You will also need to complete a merger notice form. Our merger notice template sets out the information we ask for.

Find out more about how to notify us of a merger.

Mergers intelligence committee

The CMA can also find out about mergers through our market monitoring team, known as the mergers intelligence committee. We have mergers intelligence staff who scan sources of information about mergers and present their findings to the committee.

The committee will decide which mergers that have not been formally notified should be investigated. It will also review some mergers that businesses have not already told the CMA about.

Find out more about the mergers intelligence committee.

Assessing the merger

If we investigate a merger, we consider whether it could lead to a substantial lessening of competition.

What is a substantial lessening of competition?

Competition is when businesses seek to attract customers by offering them a better deal than their rivals. Both price and non-price aspects of competition (such as service, innovation and quality) are often important parts of the competitive process.

In most cases, any reduction in competition resulting from a merger will not be substantial, as after the merger there will be sufficient rivalry remaining between those businesses who are left in the market. For example, a merger between 2 companies in a large market (with hundreds of other similar competitors) would be unlikely to affect competition across the whole market, as there would still be sufficient rivalry between those businesses who remain after the merger.

However, some mergers remove important rivalry between businesses, which can influence how other businesses in the market might behave (for example, how they price their products or whether they develop new ones). This can lead to a lessening of competition that is substantial, and could lead to lower quality goods or services, less choice or higher prices.

How we decide if there is a substantial lessening of competition

Each merger is unique, so we consider it on its own terms and with an open mind. The potential consequences that we typically consider during merger investigations include:

• if the merged business would raise its prices, reduce the quality of its goods or services, or reduce innovation efforts
• if other businesses in the market realise they could coordinate their activity so that they could potentially raise prices by not competing as much
• if other businesses in the market were prevented from buying goods or services that are a crucial input to the goods or services that they want to sell (this is called ‘input foreclosure’), or if a business stops selling its products to other customers and instead supplies only to the business it has merged with (this is called ‘customer foreclosure’)

To find out more about how we assess whether competition law concerns might be raised by a merger, read our merger assessment guidelines (CMA129).

How our investigations work

The CMA follows a careful, transparent process when we investigate a merger. To make an informed decision, we:

• make sure that the merging businesses and other stakeholders can engage with us
• gather the necessary information so we have a clear picture of the merger’s likely effect on competition

We also try to finish our investigation as quickly as possible, to reduce the uncertainty, costs and burdens for businesses. Our investigation has strict deadlines set out in law.

Pre-notification: information gathering

First, we make sure we have enough information to launch a phase 1 investigation. We call this the ‘pre-notification’ phase.

We’ll ask merging businesses to submit a merger notice form to tell us things such as:

• who they are
• what goods or services they supply and procure
• who they compete with
• how competition works in their industry

We might also ask anyone who might be affected by the merger, or who could tell us more about the relevant products, services or markets (your customers or competitors, for example), to get in touch. For example, we can publicly invite comments from these stakeholders and assess their responses. We might publish an invitation to comment in the pre-notification phase, or when we launch a phase 1 review.

We might also speak to other organisations, like competition authorities outside the UK, or other regulators.

Phase 1: initial review

At phase 1 we’ll carry out an initial review of the merger, to decide if the legal test for continuing our investigation has been met. The legal test asks whether the merger has a realistic prospect of substantially lessening competition.

Our information gathering powers

If we investigate your merger, we’ll continue to gather information from you. We have legal powers (known as ‘section 109 notices’) to gather information that will be relevant to our investigation. If we use these powers, you must provide that information in the format we ask for and by the deadline we set. We sometimes use similar powers to request information from other businesses.

To find out more about our information gathering powers, go to Mergers: Guidance on the CMA’s jurisdiction and procedure (CMA 2, from paragraph 9.5) and Administrative Penalties: Statement of Policy on the CMA’s Approach (CMA4).

We have a legal deadline of 40 working days at phase 1 (starting on the day we launch our formal investigation) to decide whether the merger needs a more in-depth (or ‘phase 2’) review.

If we do not find any competition concerns, we’ll clear the merger. If we do find concerns and think that we should refer the merger to phase 2, we’ll first give the merging businesses 5 working days to propose remedies (or ‘undertakings’) to address our concerns. If we accept these proposed undertakings, we will not refer the merger to phase 2. If we do not accept them or the businesses do not offer any, we will refer the merger to phase 2.

We’ll announce our decision by the last working day of our deadline and publish the reasons for it soon after that.

Most of our investigations end at phase 1, and the merger is allowed to go ahead without any remedies. Only a few need remedies and even fewer need to be referred to phase 2.

Find out more about our investigation outcomes.

Phase 2: more in-depth review (in some cases)

If you do not offer any remedies or, in our view, the ones you offer do not address our competition concerns, then we have a duty to carry out a more in-depth, phase 2 review. Unlike at phase 1, at phase 2:

• there is a higher legal test for us to conclude that the merger will lead to a substantial lessening of competition
• an independent ‘inquiry group’ – 3 to 5 people with a range of business, finance, economic and legal experience – leads the investigation and makes the final decision
• our deadline to reach a decision is 24 weeks, but in special circumstances we can extend by up to 8 more weeks

If the inquiry group, usually having reviewed more evidence in line with the higher legal test, decides that the merger does not raise any competition concerns then it will clear the merger.

Remedies

If the inquiry group has competition concerns, it will consider any remedies which are proposed to address them.

Remedies might involve:

• selling part of the merged business
• a legal commitment from the merging businesses to behave in a certain way

The inquiry group will only accept remedies if they address its concerns comprehensively and effectively. If the merging businesses do not offer appropriate remedies, the group can decide on some remedies (recommended by CMA experts) instead – or decide to block or unwind the merger as a last resort.

For more information about our approach to remedies and the types we consider, go to Merger remedies.

Next steps

If you’re ready to notify us of your merger, submit a case team allocation form.

If you need more information, read our other guidance and resources for businesses involved in a merger investigation.

To understand what’s changing, find out how the CMA is evolving its approach to mergers.