Brown promises 3m affordable homes - and help to buy one

He announced that three million new properties would be built by 2020 - 250,000 more than has been previously announced.

Concerns that the green belt would be built on were raised when Communities Secretary Hazel Blears said that she expected to have a "tussle" with environmentalists.

Mr Brown said that the Government would not build on protected countryside, but critics questioned whether he could realistically keep his promise.

Homes would instead be developed on publicly-owned land - such as Ministry of Defence or Department of Health sites - or brownfield land, the Prime Minister said.

Ministers will also talk to mortgage lenders about introducing more 25-year fixed-rate mortgages to protect buyers from fluctuating interest rates.

The measures are aimed at helping millions who have been locked out of the housing market by rising prices, rising interest rates and punitive stamp duty costs.

Britain's housing stock is also under unprecedented pressure from rising immigration and an increase in the number of those who live alone.

Mr Brown told MPs: "Putting affordable housing within the reach of not just the few, but the many, is vital both to meeting individual aspirations and to securing a better future for the country.

"This is a new and urgent challenge that we have got to meet.

"You are not going to solve this problem overnight, but you can make housing more affordable for a large number of people who have been squeezed out of the housing market."

However opposition MPs blamed Mr Brown's tax regime for driving homes beyond the reach of millions.

Tory communities spokesman Eric Pickles said: "Gordon Brown's tax hikes, especially punitive stamp duty and escalating council tax have helped kick a whole generation off the housing ladder.

"He has presided over falling home ownership and curtailed the right of social tenants to own their home."

Mr Brown put housing at the heart of his Government by appointing his ally Yvette Cooper as Housing Minister.

Miss Cooper, who was the minister in charge of the Home Information Pack fiasco, attends Cabinet.

The Prime Minister unveiled a range of measures to combat the crisis and to

improve the quality of existing homes. These include:

• Securing up to 550 public sites to build 100,000 extra homes;

• Streamlining the planning system to speed up the building process;

• More shared-equity projects - such as the HomeBuy scheme - to help social tenants and key workers buy their homes;

• Plans to ensure social housing meets the "decent homes standard";

• 100,000 "green" homes to be built in carbonneutral eco-communities.

Mr Brown said he would raise the annual target for new homes from 200,000-a-year to 240,000 by 2016.

A Housing Bill, to be unveiled later this year, will identify public-sector land which can be freed up to build houses and flats.

Reform of the planning system will help unlock more land.

Mr Brown said: "I can assure the House that we will continue to protect robustly the land designated as green belt."

Adam Sampson, chief executive of the housing charity Shelter, said: "Gordon Brown's commitment to build three million new homes by 2020 is a boost for families and should help young people finally get a foot on the housing ladder."

The Home Builders Federation said: "The acid test will be whether the measures in the Queen's Speech will speed up the planning system and unlock more land to let us build the homes this country needs."

Local Government Association chairman Sir Simon Milton said: "The problem has never been purely land supply, but more the lack of funding for the roads, schools and hospitals which are needed to turn soulless developments into vibrant communities."