A public spending watchdog has cast doubt on the Government's ability to deliver a major upgrade to a key rail route by the 2018 deadline.
Delays in awarding the contract to build new trains for the Thameslink risk putting the entire project behind schedule.
The problems call into question the ability of civil servants to plan complex transport schemes, according to the National Audit Office.
The NAO report found there continues to be a "robust" case for investment in the route because Thameslink is one of the capital's most overcrowded services.
A contract for the trains was supposed to have been signed off by March 2010 but the DfT only selected its preferred bidder - a consortium led by Germany's Siemens at the expense of Bombardier Transportation, which builds trains in Derby - in June 2011 and the deal has yet to be finalised.