The trustees of London’s garden bridge, including actor Joanna Lumley and the former Labour minister Lord Davies, could have breached their legal duties over the failed project, that cost taxpayers more than £40m, according to a leading lawyer.

The legal opinion comes as pressure mounts for a formal investigation into how the charity behind the abandoned scheme spent so much money without construction work even beginning.

Labour and the Liberal Democrats on Tuesday noted that the Kids Company charity received the same overall amount in taxpayer funding but faced several investigations after it closed amid acrimony in 2015.

The opinion, by Jason Coppel QC, an expert in public and procurement law, and shown to the Architects Journal, suggests there could be a possible legal claim against the trustees of the Garden Bridge Trust.

As well as Lumley and Lord Mervyn Davies, a banker and junior minister under Gordon Brown, the trustees included the PR executive Roland Rudd, the brother of Tory MP Amber Rudd, and Paul Morrell, formerly the chief government construction adviser.

Coppel said any potential breach of trustee duties was particularly relevant over the decision by the trust to sign a construction contract for the bridge despite there being a series of planning-related hurdles to overcome and the necessary private sector funds having not been raised.

The decision to press on with the construction contract led to public losses, initially capped at £16m, increasing to an estimated total of £46m by the time the scheme was cancelled in 2017.

The project, championed by then London mayor, Boris Johnson, was intended to be majority-funded by private donations. However, the bulk of the money spent came from the £60m in public funds handed to the project.

Coppel was asked by the party who commissioned the legal opinion – who is remaining anonymous – to look into a potential claim for breach of duty against the trustees over the losses.

The QC wrote that while blame could also be placed with Transport for London and others, the trustees held some apparent responsibility, in particular the decision to sign the construction contract “at a time when the trust had neither secured all of the necessary funding for the project nor the necessary rights to use the land which would be required”.

He went on to argue, however, that any legal action against the trustees would be complicated by the fact that the beneficiaries of the Garden Bridge Trust were identified only as “members of the public at large”.

Architects Journal said the trustees declined to comment, but that it is understood they believe they acted at all times in accordance with duties.

The idea for the tree-filled pedestrian link from Temple to the South Bank in the centre of London was based on an idea suggested by Lumley, and strongly supported by Johnson and the then chancellor, George Osborne.

However, after doubts grew about the planned location in an already packed tourist area, its supposed utility as a transport link, and whether the trust could raise sufficient funds and deliver the project to budget, the bridge was eventually cancelled by Johnson’s successor, Sadiq Khan.

Andrew Gwynne, the shadow communities secretary, said taxpayers deserved an explanation. He told Architects Journal: “When Kids Company failed, ministers intervened and brought proceedings against those leading the project, but there has been no interest from the government in the Garden Bridge project.”

Caroline Pidgeon, the Lib Dem chair of the London assembly’s transport committee, said Lumley and her colleagues should see “exactly the same level of scrutiny as faced by the trustees of Kids Company”.

She said: “The Charity Commission claims its hands are largely tied, yet there is now clear legal advice that the trustees might have breached their duty to act with reasonable skill and care.”

The commission cleared the Garden Bridge Trust of financial irregularities in 2017. A spokeswoman said it had not seen the legal opinion and that any new evidence “will be assessed in line with our normal processes”.