This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

International Manual

UK residents with foreign income or gains: claims for double taxation relief against UK tax: evidence - self-assessment

An individual who claims tax credit relief in their self assessment is not required to supply evidence of foreign tax paid with their completed return. The customer is nevertheless under an obligation (see INTM162540) to ensure that the appropriate supporting documentation is obtained and kept for production upon request (for example if the return becomes the subject of an enquiry).