David Cameron said the Help to Buy mortgage scheme was bringing "peace of mind" to young home owners as he insisted it was not causing damage to the property market.
The Prime Minister said the £3.7 billion initiative was helping young people "who don't have a rich mum or dad" afford to buy their own homes as well as boosting the house-building industry and the wider economy.
Mr Cameron spoke as he visited a new housing development in Kempston near Bedford where he said more than 2,000 people in the east of England have used the equity loan scheme - twice as many as in London.
Some experts have raised concerns that Government schemes such as Help to Buy are pushing house prices further out of some people's reach by fuelling strong demand in the market without the supply of properties for sale keeping up.
But Mr Cameron said: "There's no damage to the property market. This is good for the property market because young people are able to get a home of their own and get the peace of mind and security that brings.
"House prices in Britain are still 16% below the peak level they reached and the official economic forecast is for them to still be 2% below that peak level even in 2018/19."
But the National Audit Office (NAO) found there is no method in place to measure the "joined up" impact of a string of recent Government's initiatives aiming to inject new life into the housing market.
Margaret Hodge, chairwoman of the Public Accounts Committee, said she was "shocked" that the Government is ploughing billions of pounds into the scheme without fully understanding its effects.
Mr Cameron insisted, however, that the scheme had been "a huge success".
He said: "What it means is young people who can afford a mortgage payment but who don't have a rich mum and dad and can't afford a deposit, it means they can buy a home of their own.
"They can get that peace of mind and stability and security that home ownership brings.
"It also meant actually more houses are being built.
"It's good for the economy, it's good for individuals who can buy and own their homes and we should back it."
The Government is to make equity loans to 74,000 households over three years and expects to make back its investment in cash terms after 15 years and go on to recoup £4.8 billion.
But the NAO said this week the Department for Communities and Local Government (DCLG) cannot yet "robustly" quantify the benefits of the scheme.
The NAO investigation covered the first part of the Government's flagship Help To Buy programme which was launched in England last spring and not the second UK-wide phase which kicked off last October.