The charity behind London’s garden bridge project faces being sued by wealthy donors who fear their money might not be returned even after the much-criticised scheme collapsed, with a loss to taxpayers of almost £50m.

One individual donor claimed the money he gave to the Garden Bridge Trust had been “pissed down the drain by a bunch of incompetents”, and that he wanted it returned.

While £60m in public funds was provided for the planned tree-filled link across the Thames, the rest was supposed to have been given by corporate and individual private donations.

The project collapsed in acrimony last year without construction work beginning, after the trust in charge of the bridge was unable to raise sufficient funds or complete planning approvals. The loss of public money is estimated at £46m.

Private donors are owed about £7m and, according to Architects’ Journal, the trust faces a potential class action suit from those who fear the money will not be returned in full.

The Nottingham-based law firm Freeths told the publication it was acting for one donor, who had not been named. Freeths said the donor gave £250,000 to have a bench on the bridge named after them and had yet to receive the money back. The firm intends to launch a class action on behalf of them, and other donors.

Christopher Clayton, a solicitor at Freeths, told Architects’ Journal he understood donations were made on the basis they would be returned if the project did not take place. “There are a number of both private and corporate donors who generously donated significant sums of money on the same basis,” he said.

The publication also spoke to a commercial property developer who gave £50,000 to the project and who was highly critical of the way it was handled. “The money was pissed down the drain by a bunch of incompetents and I feel conned,” said Michael Gross, the owner of Sydney and London Properties.

Gross was critical of the charity’s trustees and Boris Johnson, who as London mayor was supportive of the garden bridge. Johnson has struggled to defend decisions he made, which critics say contributed to the scale of public losses.

“Boris has got a big mouth but he doesn’t deliver,” Gross said. “There should be a parliamentary inquiry and a criminal inquiry as well – the amount of money lost demands it.”

The Garden Bridge Trust, which recently published its latest accounts, is being wound up and, as part of the process, seeks £5.1m in final funding from Transport for London.

A spokeswoman for the trust said it had “taken great care to ensure that the funds it has received have been correctly applied in accordance with the legal advice it has received. In all cases where the advice confirms funds as returnable under charity law, it is the intention and expectation of the trustees that they will be, and funders have been advised accordingly.

“The process of repayment is certainly taking longer than anticipated, for reasons that are beyond the control of trustees, but we continue to work towards resolution with Transport for London. In the meantime, we thank our funders for their support and continuing patience.”