Passengers are being warned of delays on train services from Huntingdon and St Neots to London over the Easter weekend.
First Capital Connect (FCC) is running a limited service into London Kings Cross on Good Friday and Easter Saturday with all other trains stopping at Finsbury Park.
On Easter Sunday, there will be a revised timetable for trains between Huntingdonshire and the capital, with no trains going directly to London Kings Cross.
A bank holiday timetable will come into effect on Easter Monday for all FCC trains.
The changes will enable Network Rail to work on the connection between the Great Northern and the Thameslink routes as part of the £6.5billion Thameslink programme.
Keith Jipps, FCC's customer service director said: "We apologise for any inconvenience to passengers planning to travel over the Easter weekend. We have taken steps to minimise the impact of Network Rail's work on our services, by opening the line into Moorgate as well as running longer 12-carriage trains where possible.
"We strongly advise passengers to check before they travel and allow extra time to complete their journeys.
"The work being carried out is a vital part of the Thameslink Programme which will give passengers on the Great Northern route a wider choice of destinations from 2018."