The flagship Help to Buy scheme will not help Hampshire families on modest incomes climb onto the property ladder, a study has found.

Eye-watering mortgage repayment costs will continue to put home ownership out of the reach of low and middle-income families, a think-tank warned.

It is calling, instead, for more ‘shared ownership’ with housing associations, which would be “more affordable and much lower-risk” than Help to Buy.

The conclusions will embarrass David Cameron, who has championed the scheme as a way for ordinary people to achieve the dream of home ownership.

The Treasury is underwriting up to £130bn of home loans to persuade lenders to make mortgages available with a deposit of just five per cent of the purchase price.

In Southampton, where a typical terraced home costs £160,000, a buyer would have to find just £8,000 – instead of up to £30,000, as currently demanded.

But think-tank the Resolution Foundation said the problem was the scale of monthly mortgage repayments, not simply finding the required deposit. It found that in a staggering two-thirds of the country they could not be afforded by a couple with one child on a disposable income (after tax) of £22,000.

A mortgage is widely considered to be unaffordable if a household must set aside more than 35 per cent of its post-tax income for the mortgage. In Southampton a couple would need to spend 49 per cent of their available cash – or £900 a month – to repay the home loan. And that proportion would be even higher in most of Hampshire, in Fareham (53 per cent), Eastleigh (54 per cent), Test Valley (54 per cent) and New Forest (63 per cent).

In Winchester a couple with a post-tax income of £22,000 would have to put aside a staggering 73 per cent of it – or £1,337 each month – for the mortgage.

Vidhya Alakeson, the thinktank’s deputy chief executive, said: “The aspiration to own a home remains strong among millions of families.

“But the growing gap between renting and home ownership is too great for many, especially in London, but also in hotspots across the country.”

In contrast the same couple could afford a 25 per cent share of a two-bedroom home across Hampshire – spending 24 per cent of post-tax income in Southampton, for example. Furthermore, the Resolution Foundation’s study is based on property prices at the start of the year – before recent rises, which will have hiked mortgage costs further.

But Mr Cameron has hailed the scheme for “already delivering”, after 2,384 people put in offers in just the first four weeks of Help to Buy.

Earlier this month he said: “Most Help to Buy applicants are first-time buyers, young and have a roughly average household income.”