Trust chairman admits it is “a crucial time” for scheme, adding it would be “tragic” if government pulled funding plug

The Garden Bridge Trust has issued a plea to the government to keep backing the scheme after it admitted the cost of building it has gone up.

Trust chairman Mervyn Davies is asking transport secretary Chris Grayling to make sure it extends a £15m underwriting of the project by 12 months to next September.

The figure is insurance to cover anything happening to halt the project and Davies said: “Now is a crucial time for the Garden Bridge. We have faced considerable challenges but we are now on the brink of building a truly unique crossing. It would be a tragedy if the government withdrew their support now.

“The decision now rests with the DfT to extend the underwriting. We are not asking for more public money but we do need the government’s renewed backing.”

Work on the bridge had been expected to start this spring by contracting joint venture Bouygues TP and Cimolai. The Trust has already spent more than £30m of the £60m it has received in a public handout, but the Trust, who will build and run the bridge, said hold-ups over planning and land purchases meant construction will now not be completed until 2019 – a year later than scheduled.

The delay will increase speculation that the bridge, designed by Thomas Heatherwick, is on a logistical collision course with the Thames Tideway Tunnel known as the super-sewer.

A navigational risk assessment report by Marico Marine for the bridge’s engineer Arup back in 2014 said the construction timetable needs to be developed so “the bridge construction works programme does not coincide with the major Thames Tidal Tunnel construction work at Blackfriars”. The Garden Bridge is being proposed just a few hundred yards upstream from Blackfriars.

Last year Garden Bridge Trust deputy chair Paul Morrell said it wanted to start work on the bridge by January 2016 with any potential clashes with the super-sewer resolved by spring 2017.

Most of the major works on the bridge – including closing the central river channel of the Thames to build the central span – need to be finished before the major tunnelling work on the £4.2bn super-sewer starts next year.

A planning submission by the Garden Bridge Trust in May 2014 stated: “Any other works involving restrictions to vessels would have been substantially completed by spring 2017, in advance of major river movement related to construction of the Thames Tideway Tunnel.”

Davies also revealed the Trust is still short of £56m in funding – around a third of its revised total cost. It has so far raised £69m through private donations.

He added that government support would be crucial in raising the remainder of the funds needed.

“The Department for Transport and Transport for London have been valuable supporters and partners from the very beginning. They provided funding because of the clear public transport case for the Bridge, without which we would not have got the project off the ground. This funding has helped kick start the public funding drive.”

The Trust said construction will not start until land deals on both sides of the river have been finalised and all outstanding planning matters have been completed, which it said it “thought will be concluded by autumn”.