Passengers feel the squeeze as new trains are stuck in sidings

Bosses want to introduce 23 new four-car trains across the Thameslink service to ease congestion.

However, manufacturer Bombardier today revealed it could supply only 15 of the trains by March. It said the problems were caused by its suppliers facing financial difficulties.

The situation is so acute that First Capital Connect, the train operator that runs Thameslink, was told initially it might receive only seven of the planned 23 trains.

There are now fears they may not even be delivered on time in March. The Department for Transport has been in talks with First Capital Connect to discuss contingency plans in case the stock fails to arrive.

A spokeswoman for watchdog London TravelWatch said: "It is very worrying. If these are not delivered on time it will mean a reduced and more overcrowded service on what is already a very crowded railway."

Up to 130,000 people a day use the Bedford to Brighton Thameslink, which has one of the worst records for "cattle-truck" conditions. A poll last year found that half of peak-time passengers are unable to get a seat. The new trains were supposed to ensure the service could cope with a timetable that is being introduced as part of a £5.5billion upgrade.

Blackfriars station is being rebuilt as part of the renovation, which means some platforms are being taken out of action and extra trains are needed. Bombardier confirmed the company "aimed to supply" 15 new trains by the March deadline and that the remainder "would not be far behind".

A DfT spokesman said: "We are working very closely with the franchise operators to develop a contingency agreement to ensure the deadline is met so that passengers enjoy better services on the Thameslink route as quickly as possible."

A spokesman for First Capital Connect added: "We anticipate there will be no significant impact on the service compared to today."

The Evening Standard launched a campaign to make sure every commuter can get a seat after it emerged that some have been forced to stand for the whole of a 60-mile rail journey.

Britain's top 10 "standing room only" services all start or finish in London, inflicting daily misery on hundreds of thousands of passengers. Passenger group London TravelWatch is backing the Standard's campaign.

Meanwhile, rail industry sources are warning that problems with the complicated track redesign in the London Bridge area could delay the overall Thameslink upgrade programme. "Uncertainties over London Bridge may soon be putting the scheduled completion of the programme by 2014 in doubt," one source said.

Network Rail, which is responsible for the work, denied the claim.