Passengers have been promised fewer delays and more reliable services by the country's biggest rail franchise.

But the King's Lynn to Cambridge line is set to miss out on longer trains, while extended stops at stations mean longer journey times between Lynn and London from next month.

Govia Thameslink Railway's new timetable comes into force on Sunday, May 20 (2am).

Announcing the programme today, the firm said space for 40,000 more passengers into London at peak times would be added next month, a weekly capacity boost equivalent to the population of Bristol.

Major upgrades have been introduced on the GTR network, comprising Thameslink, Great Northern, Southern and Gatwick Express, which is the UK's most congested railway with passenger numbers doubling in 12 years.

Charles Horton, GTR's chief executive, said: 'A huge number of passengers will notice the benefits in terms of new journeys, better intervals between trains, improved reliability, and more capacity across a wide region.'

GTR's network-wide shake-up includes direct Thameslink services from Cambridge and local Great Northern stations to Brighton via London Bridge and Gatwick Airport.

Trains between London Kings Cross and Kings Lynn will now serve the new Cambridge North station.

But as well as missing out on the longer trains - because the platform at Waterbeach is not long enough - passengers on the Fen Line will see an extra 10 minutes added to the 1hr 40min journey to London King's Cross.

GTR's announcement said: 'With passenger numbers doubling in 16 years, the allocated stop times at many stations are simply too short to reasonably account for those getting off and on - so at 75 stations trains will stop for longer.

'Many services currently have very short turnaround times at destination stations, so the slightest delay on the route means they don't start their return journey on time and the delay multiplies – the new timetable has increased turnaround times to help.' Business leaders and the Fen Line Users Association have criticised the delay in bringing longer trains between Lynn and Cambridge. They say the problem at Waterbeach could be overcome by selective door opening.