Rail commuters face another decade of overcrowding and delays after improvements to the Thameslink service were postponed for at least another four years.

The Thameslink 2000 project would have doubled the number of trains running from central London through Mill Hill Broadway, Hendon and Cricklewood and increased the number of carriages on the route, which is one of the most congested in the country.

But those plans were postponed indefinitely last week after a public inquiry conducted by the Office of the Deputy Prime Minister found deficiencies in designs for stations at Blackfriars, London Bridge and Borough High Street, where plans to relocate the historic Borough Market had caused controversy.

"This decision came out of the blue," said Rufus Barnes, director of the London Transport Users Committee.

"People who want to preserve old buildings in Borough Market are holding modernisers to ransom. We have got to decide if heritage is more important than moving forward.

"Last week was a pretty bad week for London. Thameslink 2000 is an extremely important scheme for linking north and south London, but we should rename it Thameslink 2012."

The delay to the programme, which was due to be completed in summer 2006 after a catalogue of other postponements, will affect the prosperity of large parts of the borough. A new Thameslink station planned for Cricklewood is the jewel in the crown of the Cricklewood regeneration scheme adjacent to Brent Cross shopping centre.

Passengers in Mill Hill have been expecting longer platforms to take new 12-carriage trains, increased frequency from two to eight trains an hour (24-an-hour in central London), better cross-London services and an expanded network as far north as King's Lynn in Norfolk to Littlehampton and Eastbourne on the south coast.

After five years of lobbying for speedy progress, Hendon Labour MP Andrew Dismore was philosophical about the delay.

He said: "It is obviously disappointing but there are deficiencies that have got to be addressed. It is not dead, it just needs more work."

Thameslink 2000 communications director Martin Walter said: "If they get their sleeves rolled up and get on with it, hopefully they will find a solution sooner rather than later. We will be pushing very hard to accelerate the process."