Parliament’s spending watchdog is to investigate why chancellor George Osborne decided to give £30m of taxpayers’ money to a deeply controversial garden bridge over the Thames.

The chair of the powerful Public Accounts Committee, which oversees all public spending, has asked the National Audit Office to examine the “rationale” behind how the garden bridge received the lump sum and also how the government intends to “maintain oversight” over its £30m funding.

Meg Hillier, the former Labour Home Office minister, in correspondence seen by the Observer dated 3 September 2015, states: “They [the NAO] will report to me any concerns which may arise from their inquiries.”

Osborne pledged £30m of public money for the project and then wrote to the London mayor, asking him to “do the same,” a request Boris Johnson obeyed by also giving a £30m grant.

The garden bridge is facing a mounting backlash with critics saying the £175m project is a misuse of public funds, claiming that rather than providing a public crossing, it risks creating a privately managed tourist landmark.

Pressure will increase on 17 September, when the Greater London Authority’s oversight committee will scrutinise how procurement contracts relating to the garden bridge were awarded, a process won by designer Thomas Heatherwick and which since has become an issue of controversy.

Thomas Heatherwick has won the design contract, despite being the highest bidder. Photograph: Nick Cunard/REX

Internal Transport for London (TfL) documents show that Heatherwick scored more highly than other architectural firms in the crucial “relevant design experience” category, despite having built just one bridge compared to competing firms who had designed up to 25 crossings.

Later, it also transpired that Heatherwick submitted the highest quote of three bidders in the competition to design the concept for the bridge, but was said by TfL to be the most “economically advantageous”.

Of the three firms invited by TfL to tender, Heatherwick’s proposal was £173,000, three times more expensive than the £49,939 offer by Wilkinson Eyre, and more than 11 times the £15,125 offer by Marks Barfield.

Concerns over the procurement process forced former commissioner of Sir Peter Hendy to order a review of the procurement process for the project’s design.

Despite the injection of public funding, the charity behind the project, the Garden Bridge Trust, still requires a significant amount of financial backing and among its revenue-raising initiatives are naming opportunities for items on the bridge.

Michael Ball of campaign group Thames Central Open Spaces alleged that such initiatives illustrated flaws in the business case of the project.

He said: “The fact that the Treasury accepted it is deeply worrying – it does not comply with the most basic requirements of the Treasury’s Red Book. How many other publicly funded projects should the National Audit Office be looking into because George Osborne and his people failed to do their job?”

Boris Johnson has controversially guaranteed that the estimated £3m annual running costs of a garden bridge over the Thames would be met by public money, if private funds were insufficient.

TfL says that there is no need for a fresh appraisal of the procurement process, saying it is “satisfied a robust and proper process was followed to award this contract”. A spokesman added that the initial tender was issued to three design firms “as at the time there was no approved TfL architectural framework in place”.