Passengers are being warned not to travel to London by rail over two weekends when trains will be cancelled on part of the network.
Improvement works are taking place every weekend in January and February, with some disruption to services.
Great Northern, which runs services between King's Lynn and London King's Cross, is advising people not to travel to the capital by rail on January 25 - 26 and February 29 - March 1.
Trains from Lynn will only be running as far as Hitchin.
Network Rail is renewing track, signalling and overhead wire equipment on the approach to King's Cross.
Lines are being re-laid in a new layout, with a disused tunnel being reopened, allowing for six tracks into the station instead of the current four.
Network Rail said services on other weekends are likely to be very busy and passengers are advised to reserve a ticket for their journey where possible.
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