Brighton commuters will have to wait even longer for new train carriages.

Contracts for a fleet of 1,200 new train carriages for the Brighton to Bedford line are not set to be signed until the autumn, meaning Sussex commuters will have to wait even longer to see new carriages and longer trains.

The new carriages were originally planned to be in use by 2018.

Last month Rail Minister Theresa Villiers announced plans to build new carriages and run longer trains, increasing capacity on the Thameslink route, which is one of the most overcrowded in the country.

A consortium led by Siemens was named as the preferred bidder, in a deal expected to create hundreds of construction and maintenance jobs at the firm’s Crawley base.

However, the company may now have to wait until the autumn for final approval of the deal, which was expected to have been completed before the start of the Olympics .

A Department for Transport spokesman said: “We are making progress with Siemens and their consortium and expect to reach financial close by early autumn.”

Steve Scrimshaw, head of Siemens’ UK train division, told a national newspaper that the firm was already making “good progress on the trains” and said some “preliminary engineering and manufacturing work” was already taking place.

If the deal goes ahead as planned, Siemens plans to set up a new maintenance depot at Three Bridges, in Crawley, but still needs to seek planning permission from Crawley Borough Council.

The first stage of the Thameslink programme has already started, including longer 12-carriage trains on the Bedford to Brighton route.

Crawley MP Henry Smith has been a major supporter of the plans, which are expected to bring more than 650 jobs to the town.

He said: “It demonstrates that Crawley has a skilled workforce, great communications and is a good place to live.”

The first carriages were due to be completed by 2015, with the full fleet in use by 2018.

Existing Thameslink carriages will be used to relieve overcrowding on rail routes in other parts of the country.