London mayor Sadiq Khan has frozen a building project for the Thomas Heatherwick-designed Garden Bridge to ensure no more public money is committed to the £175 million scheme.

Khan has temporarily blocked construction at Temple tube station, where works were due to start to strengthen the existing structure so it can withstand the weight of the new River Thames crossing.

The decision was prompted by concerns that £3 million of taxpayers' money would be used to fund the works – because the mayor has pledged that no more public funds will be spent on top of the £60 million already committed.

He has promised to make the project "more open and transparent" than it was under former mayor Boris Johnson, despite the risk of delaying the scheduled 2018 opening.

"The previous mayor first approved plans for enabling work to prepare Temple tube station for the arrival of the Garden Bridge two years ago in the summer of 2014, but final authorisation was only provided in March this year, two months before the mayoral election," said a spokesperson for the mayor's office, adding that "this enabling work has since been suspended".

"Sadiq Khan has been clear that no new public funds should be committed to the Garden Bridge and he has pledged to make the project more open and transparent – standards that were not always met under the previous administration."

The move is the latest in a series of blows for the controversial project, which was conceived by British actress Joanna Lumley.

The proposal has already been called into question over funding sources and the fairness of the procurement process that led to Heatherwick being appointed as designer, as a result of a lengthy investigation by the Architects' Journal.

But the Garden Bridge Trust – the non-profit organisation behind the project – has insisted that the cost of the enabling works at Temple has been factored into the project's budget and that no extra public funds will go towards it.

It also rejected claims that it had asked for a loan to cover the work from Transport for London, the local government body overseen by Khan.

"London Underground Ltd is doing a range of work for the Trust to enable the Garden Bridge to land on the top of Temple tube station on the north bank," a spokeswoman for the Garden Bridge Trust told Dezeen.

"This is essential work which involves strengthening of the roof, enabling the construction of a new lift shaft for the north landing point, diverting cabling out of the area of construction, and foundation works."

According to the Trust, a first payment for the work has already been made to London Underground Ltd. It said the Temple works were likely to start soon.

Billed as a "beautiful new garden floating above the River Thames", the 367-metre-long Garden Bridge is proposed across the Thames between Temple and the South Bank.

Construction of the structure must complete in late 2018 to avoid clashing with the installation of a major new London sewer.