Opposition politicians at City Hall have challenged Boris Johnson’s claim to have secured government backing for a new rail link in east London.
The mayor said a Budget pledge from George Osborne would allow a major housing development to go ahead at Barking Riverside.
City Hall officials said a £180m extension to the Gospel Oak to Barking railway line would lead to the creation of a new “garden city” on the banks of the Thames.
But Liberal Democrats on the London Assembly said the Treasury was only committing itself to “develop proposals”.
Labour described it as a “plan for a plan” without any real allocation of money.
A spokesman for the mayor admitted there was a funding gap but said a deal with the Treasury was expected in the Autumn.
Barking Riverside is said to be the largest single housing development site in London with the potential to create a new town with a population of 29,000.
The development of 11,000 homes, which began in 2010, is a joint venture between City Hall and a building firm, Bellway Homes.
A £700m plan to extend the Docklands Light Railway to Barking Riverside was scrapped by Boris Johnson shortly after he was elected in 2008.