Fears raised over meeting Thameslink deadline
Delays in selecting the trains to be used for the government's Thameslink upgrade "raise questions" about whether the project as a whole will come in on schedule, a spending watchdog says.
A contract was due to be signed by March 2010 but the deal with German firm Siemens has yet to be finalised.
The National Audit Office said this meant the multi-billion-pound scheme's "feasibility" was in some doubt.
The government said it was confident it would be completed in 2018, as planned.
The Thameslink route, which runs from Brighton to Bedford, via London, is being improved, with longer trains and platforms being introduced to reduce congestion.
A £1.6bn contract to supply trains was due to have been signed by March 2010, but the Department for Transport did not choose its preferred bidder - Siemens - until June 2011.
The German company was chosen ahead of Derby-based Bombardier Transportation.
The National Audit Office (NAO) report found there was a "robust" transport case for investment in the route as it was very overcrowded.
But the procurement delay could complicate other parts of the project and was partly behind stalled plans to re-let the franchise, it added.
The report said: "The delays raise questions about whether the department underestimated the scale of work, time and skills and capacity it required to negotiate a complex PFI deal."
Amyas Morse, head of the NAO, said: "Our principal concern is around the delay in agreeing the contract to build new trains which raises questions about the feasibility of delivering the whole programme by 2018."
However, he said there had been "good progress in delivering the first stage of the infrastructure part of the programme on time and under budget, which the department now needs to build on".
Rail Maritime and Transport union general secretary Bob Crow said the report confirmed the "chaotic mismanagement of the Thameslink upgrade right up to cabinet level".
He added: "In just over a week's time we will mark the second anniversary of the award in principle of the Thameslink contract to Siemens in Germany at the expense of Bombardier in Derby.
"After two years of government lies and broken promises the contract remains unsigned and train building in the nation that gave the railways to the world is on a knife-edge. This scandalous political incompetence must end now."
A Department for Transport spokesman said the procurement process was "complex" and it was "important that we get it absolutely right".
He continued: "However, we are in the final stages of the process and we expect to conclude the deal shortly.
"We have an experienced senior team in place and are confident that we will be able to deliver the remainder of the project on time and on budget."
Shadow transport secretary Maria Eagle said: "It is bad enough that ministers decided to build the new Thameslink trains in Germany, but staggering that they can't even be sure they will be delivered on time."
She added: "The criticism that ministers have failed adequately to staff this vital project suggests that the lessons of the rail franchising fiasco have simply not been learned."