A watchdog has said the roll-out of new rail services to ease travel between the north and south of the country through London has been delayed up to a year.
There had been plans to install a metro-style timetable of around 24 trains per hour in peak time by the end of 2017.
However the National Audit Office (NAO) has revealed the plans for this have been pushed back.
To compensate, Thameslink services will increase to 18 trains per hour next May and 20 per hour in December 2018.
The NAO said the decision was made after the Department of Transport and National Rail did not make suitable arrangements to launch the new services.
A spokesman for Network Rail defended the Thameslink Programme would provide more reliable journeys for users.