Thousands of commuters are facing disruption to their journeys on Monday because of work on a new station at St Pancras for Thameslink.
Work on a new station at St Pancras lasts until May 2005
During the work Thameslink trains from the north will end at St Pancras while services from the south will terminate at King's Cross and Blackfriars.
Trains will not be stopping at West Hampstead and Kentish Town between 0730 BST and 1030 BST from Monday to Friday.
Thameslink say about 20,000 passengers will be affected between those times.
A Thameslink spokesperson said: "Service alterations are necessary whilst work is carried out on the Channel Tunnel Rail Link and the new Thameslink station."
He said Thameslink have recruited more drivers and support staff and provided extra rolling stock throughout the work on the new station.
There will be extra passengers walking between King's Cross Thameslink and St Pancras as a result of the split Thameslink service.
Passengers using the station will receive a credit card-sized card, known as the Thameslink Travel Buddy, to help plan their journey.