The Garden Bridge Trust has pushed back the deadline for filing its annual accounts by five months.

The charity has changed its year-end from October (the month it was founded) to March, meaning it will be filing 17 months of accounts in one go next spring.

It strongly denies this is anything to do with transport secretary Chris Grayling’s impending decision about whether to extend the government’s £15 million underwriting guarantee.

Until it changed its year-end it would have been due to file its annual accounts on Friday. But it needs Grayling to approve the extension before it can do this, in order to demonstrate it can meet its liabilities if the project were scrapped before construction begins.

A spokeswoman for the trust said the “realignment” of its year-end had nothing to do with Grayling’s decision but merely brought it in line with other charities.

The trust wrote to Grayling in July asking him to extend a £15 million financial guarantee – which is due to expire at the end of next month – by a year.

He has since demanded detailed financial information about the £175 million project. He has asked for a breakdown of how £36.5 million of public money has already been spent. And he wants to know how the final £30 million needed for the Heatherwick-designed project will be raised.

The trust said it was working closely with the DfT and had provided requested information.

It emerged earlier this summer that most of the cash that has already been spent came from the pot of public money given by the Treasury and TfL. The trust refused to tell BD how much private money had been spent, citing “commercial confidentiality” - though it is unclear who could be compromised by the release of such generalised information.

Asked why it had spent public money ahead of private donations, it said it was “a condition of grant money that the public money allocated is used for pre-construction and construction work”. It said that more of the £83 million of private money would be unlocked once construction began.

Meanwhile, the trust has also confirmed that it has decided to postpone the start of construction until all remaining land and planning issues have been resolved, something it hopes – but can’t guarantee – will be sewn up by the autumn.

The spokeswoman said they foresaw no potential clashes with Thames Water’s super sewer construction programme.

A statement said: “The trust has made great progress on the Garden Bridge this year with the discharge of planning conditions, ground investigation work taking place, agreeing logistics and a construction programme, reaching agreements with key stakeholders involved in work on the Thames, as well as strong progress on private fundraising.

“There have however been some hold-ups to the project and we will not start construction until we have concluded land deals on both sides of the river, and completed all planning matters. We aim to have done this by the autumn. We have been in extensive dialogue with the Thames Tideway Tunnel to ensure that our respective works are fully integrated with no construction clashes.”

A DfT spokesman refused to be drawn on the information Grayling has asked for, saying only: “The trustees of the Garden Bridge have asked for an extension to the financial support the government is providing to the project. Ministers are considering the request.”