Only a "lucky few" are able to take advantage of the Help to Buy scheme in England because they are being priced-out, a BBC investigation has found.
Under the scheme, the price of a house cannot exceed £250,000, or £450,000 in London. But the investigation found that starter homes exceed the price cap is many areas across England.
The government says the cap allows the scheme to focus on first-time buyers. But the Help to Buy Isa, which was launched last year, only gives savers a £3,000 bonus on their deposit if the price of the house is below the cap.
As a result, housing charity Shelter said it only helped "the lucky few".
Using average prices in around 1,000 local areas on property site Zoopla, the BBC found that a two-bedroom house exceeds the £250,000 cap in 28% of areas outside of London. In the South East, 67% of areas exceeded the cap, 61% in London, 61% in the South and 53% in the East.
However, in areas like the North East, the Midlands, Yorkshire and Lincolnshire, less than 5% of local areas exceeded the price cap.
For an average two-bedroom flat in London, the average price exceeded the cap in two thirds of boroughs, with one-bedroom flats exceeding the cap in a third. Just 10% of three-bedroom homes in London fell below the £450,000 mark.
Campbell Robb, chief executive of housing charity Shelter, said: "Soaring housing costs have left millions of people stuck in a rent trap and struggling to save anything towards a home of their own.
"The Help to Buy ISA only helps the lucky few who are better off, or able to live with their parents while they save for a deposit. For the vast majority of renters who want to move forward in life and put down roots, this scheme brings them no closer to that dream.
"If the government genuinely wants to help the nation's renters get a foot on the housing ladder, it needs to look beyond quick-fix schemes, and invest in homes that people on ordinary incomes can actually afford."
A Treasury spokesman said: "The Help to Buy Isa scheme was introduced to support those that are struggling to save enough to get onto the housing ladder. The property price cap allows the government to target support at those who are saving to buy their first home."
Personal finance expert, Martin Lewis, said people should still consider opening a Help to Buy Isa. He said: "It is a cash giveaway from the taxpayer.
"Even if you don't end up using it to buy a house, you still have savings with a very favourable rate of interest."