£22m gap in Garden Bridge funds - Newsnight investigation

The tree-covered Garden Bridge would span the Thames between Temple and the South Bank

The designer of the Garden Bridge project in London has made an impassioned defence of the proposal - arguing the project has been turned into a political football.

Speaking to BBC Newsnight, Thomas Heatherwick said scrapping the bridge would indicate that "we have suddenly have no confidence in ourselves".

The proposal for a new bridge over the Thames has been beset by controversy.

Newsnight has learned the funding gap is £22m more than previously thought.

If the bridge is ever going to be built, the team behind it need to raise £175m.

They have £60m of taxpayers' money, leaving £115m to come from the private sector, companies and individuals.

But Newsnight has learned that several funders have pulled out over the last year, and according to figures provided by the Garden Bridge Trust itself, what had been described as a £30m shortfall is actually in the region of £52m.

Asked about the discrepancy, a spokesperson for the project said: "Last year a small number of pledges made by interested organisations did not progress to formal funding contracts."

Transport for London have conceded that the procurement process was neither as open or fair as it should have been, and in July, Newsnight reported that the government is deciding whether to continue to underwrite it.

Newsnight analysed the total amount of money raised for the project

The Garden Bridge was conceived of by the actress Joanna Lumley as a memorial to Princess Diana. It was championed by Boris Johnson as London mayor and by George Osborne as chancellor.

But Sadiq Khan, the current mayor of London, has said there can be no more support from Transport for London or the Greater London Authority beyond the £30m they are contributing. He also said he believes it would be more expensive to cancel the project than to continue.

The plan is for the bridge to run between Temple Underground Station and the South Bank, and for it to be covered in trees and shrubs.

Thomas Heatherwick, who also designed the Olympic cauldron and London's new Routemaster bus, defended the scheme, and said politics must not get in the way.

Plans propose more than 270 trees and 2,000 shrubs would be planted on the bridge

"Money has been spent to get this far. It is ready to go, and it is important that our society doesn't show that we suddenly have no confidence in ourselves... that we don't suddenly seem like, 'Yep, we've had a political turmoil, now we suddenly close up and we're just going to go backwards.'"

"There's all sorts of people who want to get their little agenda... How can it possibly be a bad thing to stitch the city together better, to create new public space that we have never had before, new views for all of us."

The Garden Bridge was first proposed in 2012 and is expected to open in 2018. However, given the delays already experienced by the project, this date has been thrown into doubt. The Garden Bridge Trust has previously said the bridge will take 32 months to build.

Mr Heatherwick said it was important that all sides "hold their nerve".

"This project will be finished three years from now and we'll be walking across there with our children, with our grandmothers... The best view of the city is from the centre of the river, where the buildings get out of the way."

There is more on this story on BBC Newsnight at 22:30 on BBC Two - or you can catch up afterwards on iPlayer.