Rail engineering work which was already over-running has been extended by another day and will now affect some trains on New Year's Day.
Replacement bus services are in place
The works between Coventry and Northampton mean that West Coast Mainline services are being amended, with delays expected.
Passengers travelling between London Euston and Scotland or the north west of England will be affected.
Services had been expected to return to normal on 1 January.
Network Rail said work was still not complete.
A spokesman for the company told the BBC News website: "We have put in all the extra resources that we possibly could, but things just haven't gone as well as we hoped."
The firm said that as passenger numbers were lower over the festive period it was essential to carry out a large part of the £8bn upgrade of the line then.
Virgin Trains has already complained that it was not given enough warning about the engineering works which started on New Year's Eve.
Virgin passengers are being warned they will have to take a connecting bus service between Birmingham International, Coventry and Northampton.
London Midland services are also being replaced by road transport.
Chiltern Trains, which run between Birmingham Snow Hill and London Marylebone, has said it will accept tickets purchased from other train companies but has warned that with the additional passengers services are likely to be busy.
Normal train services are expected to resume on 2 January.
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