Boris insists cable car plan is viable

He said: "We've got to be honest. There's a chance that we can do it, but the issue is finance. We're going ahead with the planning, we've got all the support we need, but in tough economic circumstances we need to raise the money privately.

"There has been no deal done yet, but we're working very hard on it. Obviously for 2012 there comes a moment when time becomes very critical. We're working very hard on that too."

The Mayor was adamant there was a very good business case for building the cable car link, despite doubt being cast on its viabilty after the Olympic period.

He said: "People shouldn't underestimate the volume of people it can carry. A cable car is equivalent to one bus route with a substantial number of buses on it. It's not insignificant as a people mover.

"The ambition is to link the biggest and most popular live music venue in the world with all the development and things happening at the Royal Docks. That makes long-term sense."

But there are fears over the commercial viability of the £25million project. Mr Johnson denied Transport for London will have to bail out a private operator to keep it running if passenger numbers were low.

He said: "There is a very good business case, but plainly we will be keeping a very beady eye on the bottom line and making sure we don't invest in something that doesn't have a long-term future."

More river crossings for east London, including a new tunnel, are about to be announced. Mr Johnson refused to elaborate on his plans.

He said: "We'll be making a series of announcements pretty shortly about a package of things we want to do. They will include a tunnel, another fixed link, more work on another bridge in the Gallions Reach area, and further upgrading of the Woolwich Ferry and the cable car.

"There's a lot of work going on, but I don't want to get into detail at this moment."

They could prove a legacy for the Mayor, and he is still thinking about standing for re-election in 2012.

He said: "I think if I still think things are going well towards the end of this year I'd be crazy not to have another go at it."