Work to build a £50m cable car across the River Thames in London, linking two Olympic venues, will begin this summer.
Transport for London (TfL) said it was "striving" to complete the connection before next year's Games but warned this would be "an extremely challenging timeline for a complex project".
It will run from the O2 arena in North Greenwich to the Excel exhibition centre at the Royal Victoria Docks.
It will be part of the Oyster card network but fares have yet to be set.
Last year TfL said it would not be funding the cable car but now it has decided to pay for the project up front.
This was to help its "quick delivery", a spokesman said.
While the company would be trying to recoup its costs, it could not guarantee that no money would have to come from taxpayers, he added.
'Not serious solution'
The consortium which will build and run the cable car is to be led by Mace.
The firm worked on the London Eye and is currently constructing the Shard Tower at London Bridge, south-east London.
London's mayor, Boris Johnson, said: "Gliding serenely through the air across the Thames will provide a truly sublime, bird's eye view of our wonderful city.
"This innovative airborne travel link will be a vital component in the ongoing renaissance of a vibrant easterly quarter of the capital, providing a much-needed river crossing."
But a Labour member of the London Assembly, John Biggs, argued the cable car was "no substitute" for a bridge across the Thames.
"I've no objection to what will be a fantastic attraction, but this is not a serious solution to the real transport needs for the area, and no substitute for what we really need - a new road crossing."
And the project's cost was queried by Caroline Pidgeon, who leads the Liberal Democrats on the assembly.
"The mayor must now come clean and state why as recently as July 2010 he was claiming the cable car would cost £25m, but already its projected cost has doubled."
She also called on Mr Johnson to explain why he "was boasting" of a privately-funded cable car last year, when it was now to be paid for by TfL.