Princess Beatrice at Harrods fundraising gala for Garden Bridge

According to the Garden Bridge Trust the event raised £257,000, which adds to the £127 million already pledged. Of this, £17 million is from individual donors, £10 million from companies including Citigroup and ICAP and £40 million from major charitable trusts including the Weston Foundation and the Sackler Foundation. The initial £60 million was pledged in 2013 by Transport for London and the Department for Transport.

Earlier this week Mayor of London Boris Johnson was questioned on LBC radio about his controversial decision to provide a guarantee to Lambeth and Westminster Council that the bridge's maintenance costs will be underwritten if the trust fails to raise enough money.

The Mayor repeated his previous assurances that the guarantee will cost taxpayers "nothing" because the bridge trust's income is forecast to exceed its outgoings.

"We've been very firm with the Garden Bridge Trust and said that they have got to be in a position to fund this themselves," he said.

Mr Johnson added: "In the end, a huge project in the centre of the city cannot be done without the support of the municipal authority and the city itself."

This week the Garden Bridge was shortlisted in the transport and infrastructure category of the New London Awards for architecture and the built environment.

Meanwhile opponents of the Garden Bridge have launched a competition called A Folly for London to find "the most absurd, illogical, egotistic and obtrusive designs" for the green space on the South Bank which would be replaced by the Garden Bridge's southern landing.