Network Rail has been fined a record £14m by the railway regulator, after engineering work over-ran during the Christmas and New Year period.
But the company says that without further delays, it risks not meeting a December deadline for finishing work to the West Coast Main Line at Rugby.
Passengers now face extra disruption through the summer, as lines are closed in order to complete the work.
Network Rail says the push will enable it to finish the project on time.
The Rail Regulator fine relates mainly to major engineering work at Rugby in late December and January, which over-ran by four days, severing the West Coast Main Line and causing massive disruption.
Network Rail’s chief executive Ian Coucher admitted at the time that the work had been delayed because specialist workmen failed to turn up for work.
There was more disruption in London when late-running work at Liverpool Street station forced it to close as people tried to return to work after the Christmas break.
Thousands of passengers were affected.
The Office of Rail Regulation (ORR) said the fine – the largest ever levelled on a rail company – was to reflect “the serious nature of this breach, the impact it had already had on passengers and rail freight users and the need for the company to take urgent action to improve its approach”.
The ORR has also ordered Network Rail to provide a clear plan of how it intends to complete the upgrade of the West Coast Main Line.
The company already planned line closures over Easter and bank holidays, and has now pinpointed 13 extra weekends during the summer, when passengers will be forced to find alternative routes.
The lines will be closed between Saturday lunchtime and Sunday lunchtime.
The operations of 12 train companies depend on the project. They include Virgin, which is planning to launch a major marketing campaign in November to sell faster journey times on the modernised line.
The ORR fine comes on the day that the chairman of Network Rail, Sir Ian McAllister, is officially knighted by the Queen.
At the time of the disruption in January, Sir Ian declined to come into work because he said he would “only get in the way.”