The charity behind a plan to build a garden bridge across the Thames in London spent £53.5m without even beginning full construction, final figures for the abandoned scheme have revealed.

Of the total spent by the Garden Bridge Trust, £43m was public money – £24m from Transport for London and £19m from the Department for Transport, TfL said on Wednesday.

The rest of the money came from donations to the trust, or fundraising activities.

The final sum has been reached after an official review into the costs of ending and winding up the project, which collapsed amid acrimony in August 2017. The review decided that a final £5.5m in DfT funding should be given to the trust as part of the cancellation agreement.

Announcing the figures, TfL said the eventual cost was significantly lower than it could have been. However, serious questions remain about how the project was able to spend so much money, and the extent to which the then London mayor, Boris Johnson, backed the scheme.

The figures show that 40% of the money, £21.4m, went to the French-based contractors Bouygues. The trust faced significant criticism for signing a construction contract before all the stipulated conditions for work to begin had apparently been completed.

Johnson signed a directive as mayor in 2016 which watered down some of the conditions that had to be met before more public money was released. Questioned by the London assembly last year on why he did this, Johnson said he could not remember.

Initially devised from an idea by the actor Joanna Lumley and designed by Thomas Heatherwick, the bridge was meant to be a tree- and plant-filled oasis across the river from Temple to the South Bank.

Artist’s impression of the proposed bridge. Photograph: Heatherwick Studio/PA

However, critics said the central location, in an already tourist-heavy part of the capital, risked overcrowding, and argued that any supposed transport benefits would be limited.

As the trust struggled to agree on a final planning arrangement with the community housing trust on whose land the south side of the bridge was intended to be built, worries grew about soaring costs and a poor business case for future costs.

In the end, the trust spent more than 70% of its £60m public money allocation without building anything. Johnson’s successor, Sadiq Khan, eventually ended the scheme by refusing new funding.

As part of its review, TfL sought legal advice about whether the charity’s trustees might have breached their legal duties. It concluded there was no reasonable prospect of getting back any of the money.

While £11m of private money would not be returned, documents released by TfL on Wednesday gave details of other donations which would be sent back because of conditions placed on the gift.

These sums include £2.5m from the philanthropic group set up by the US media tycoon and politician Michael Bloomberg, and £2,200 that a private donor paid in a fundraising auction for “breakfast on the bridge”. The document says: “It seems clear that, contractually, he is entitled to his breakfast or his money back.”