The work had been due to start this weekend, but has been postponed for operational reasons.
The A1 southbound between the A428 and Blunham will now be closed from 9pm on Friday 31 October until midday on Sunday 2 November.
The road will be closed to allow the existing roundabout to be connected to the new section of roundabout which has been built.
There will be a clearly signed diversion route via the A428 (from the Wyboston junction) and Barford Road re-joining the A1 at Blunham.
There will also be overnight lane closures on during the week commencing 3 November on the A1, both northbound and southbound and the A421.
Drivers intending to travel on the A1 are advised to plan their journeys and allow some extra time to reach their destinations.
The scheme to ease congestion at this important roundabout is progressing well and is on track to complete early next year.
It is part of the Government’s £317million pinch point programme, which aims to boost local economies by reducing congestion and delays at key locations.
The pinch point programme was developed with Local Enterprise Partnerships, and each of its 123 road improvement schemes is designed to boost local economies, reduce congestion and improve safety on the strategic road network in England. It is part of the Government’s long term economic plan, and was announced in the Chancellor’s Autumn Statement in November 2011.
Live traffic information is available on the Highways Agency website and mobile app, and drivers can also listen to traffic bulletins on local radio for updates.
Members of the public should contact the Highways Agency Information Line on 0300 123 5000.
Journalists should contact the Highways Agency press office on 0844 693 1448 and use the menu to speak to the most appropriate press officer.