This service accounted for much of the 14 per cent overcrowding in the morning rush hour on the route, revealed in official figures published yesterday. Every morning 5,415 people travel on trains designed to take just 4,746.
Overall, Thameslink suffered 7.1 per cent overcrowding across its network. The next worst offender, South West Trains, had 3.9 per cent.
Thameslink said it was the "victim of its own success". Passenger numbers have risen by 50 per cent since privatisation, with 12 per cent more commuters in the last year alone.
It said thousands of passengers flocked to its core route, from Bedford into central London and out the other side to Brighton, as it cut fares and improved performance.
Railtrack has pledged to spend pounds 800m on the Thameslink 2000 project to double capacity on a bottleneck just south of the Thames. But the project is six years behind schedule and will not be complete until 2006.
Thameslink yesterday said it could solve the problem within 14 months by building more trains and buying space on the parallel Midland Main Line to run more services. But it said the Government must extend its seven-year franchise to make the investment worthwhile.
Euan Cameron, Thameslink managing director, said: "If we do nothing then within three years we will have a major problem north of the river. Waiting for Thameslink 2000 is wholly unacceptable."
Rufus Barnes, director of the capital's official watchdog, the London Regional Passenger Committee, said commuters suffered "horrendous problems" but said they were not all Thameslink's fault.
He said the amount of investment needed to meet the Government's targets for getting commuters out of their cars and on to public transport was beyond the resources of the industry.
South West Trains said passenger numbers had risen 24 per cent since it won the franchise in April 1996. It said overcrowding increased by just 0.1 per cent over the last year, despite a 9.8 per cent passenger growth, because of its investment in extra trains. But a spokeswoman said: "Major infrastructure investment is needed such as lengthening platforms."
Railtrack last week suggested introducing double-decker trains and longer platforms but made no firm commitments, saying other companies would have to share the cost.