More than half of people using the government’s help-to-buy loan scheme could have purchased a home without support from the state, according to Whitehall’s spending watchdog.

In a critical report into the government’s flagship scheme to help more people get on the property ladder, the National Audit Office said about three-fifths of buyers could have bought a home without the support.

The scheme, launched five years ago by the former chancellor, George Osborne who was hoping to boost housing supply amid a national shortfall, also pumped up the profits of Britain’s biggest housebuilders, the NAO said.

According to the report, which used figures supplied by the ministry of housing, communities and local government, as few as 37% of buyers said they could not have bought a property without the support of help to buy, meaning the scheme has led to as few as 78,000 additional sales of new-build properties since 2013.

Ranked as the government’s most expensive initiative to boost housing supply, with about 211,000 loans worth £11.7bn made to house buyers over the past five years, the NAO said help to buy had achieved its goals of increasing home ownership and the construction of new homes.

About 38% of all new-build property sales have been supported by the loans, equivalent to about 4% of all housing purchases during this time. In a sign that the scheme has helped bring new homes to the market, the number of new-build properties sold has risen from 61,357 a year to 104,245 a year in 2017-18.

However, the scheme, which offers buyers loans worth up to 20% of the market value of an eligible new-build property – and 40% in London – with zero interest for five years, is not means tested.

Although primarily supporting first-time buyers, help to buy is also open to existing homeowners. About a fifth of people using the scheme already own property, while more than 8,000 people who benefited from it had household incomes of more than £100,000, the NAO said.

The scheme has proven controversial as housebuilders’ profits have dramatically increased since the introduction of the scheme, benefiting company bosses and shareholders.

More than £2.3bn in dividends was paid out by the nine biggest firms in their most recent financial year, while the housebuilder Persimmon – half of whose sales were through help to buy – offered its chief executive, Jeff Fairburn, a bonus worth £110m. He was asked to leave amid public fury last year after the bonus was scaled back to £75m.

Gareth Davies, head of the NAO, said: “Help to buy has increased home ownership and housing supply, particularly for first-time buyers. However, a proportion of participants could have afforded to buy a home without the government’s help.”

He said it was not yet possible to assess whether the scheme had delivered value for money for taxpayers because the NAO needed to observe the longer-term effects on the property market. However, he warned that the government faced a challenge to wean the property market off the scheme with as little impact as possible on its plan to build 300,000 homes a year from the mid 2020s.

By 2023, the amount loaned under help to buy is expected to reach as much as £25bn, while the government forecasts it will recover its investment by 2031-32 before making profits thereafter as people repay their loans.

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Kit Malthouse, the housing minister, said: “Help to buy has been genuinely life changing for first-time buyers across the country, helping them secure their first step on the property ladder.

“Not only has it supported more than 170,000 first-time buyers, it has increased home building by nearly 15%, and is set to make a profit for the public: it’s been a win-win.”

He said that from 2021 the scheme would be extended and strengthened to make it exclusively for first-time buyers.