Research from the Campaign to Protect Rural England (CPRE) found that in the past year there has been an increase of 50,000 houses planned to be built on England’s green belt, a 25% rise.

This is despite a pledge from the Conservative Party in last year’s general election campaign to protect it.

There are now 275,000 planned builds for the green belt, almost 200,000 more than when the government introduced its planning reforms in March 2012 and almost double the 149,000 in what the CPRE called Labour’s “unpopular” 2009 regional plan.

Using data from draft and adopted local plans, CPRE says its ‘greenbelt under siege’ research “is the latest finding to challenge the Government’s commitment to the green belt”.

It warns that local authorities under pressure from the government to meet housing targets are changing the boundaries of local greenbelt areas to enable more construction to take place “at the fastest rate for two decades”.

The research found that in the year to March 2015, 11 councils approved boundary changes for further development and almost 350 hectares of green belt land was changed to residential use, according to official figures. This represents 7% of all new residential land and is one of the highest rates since 1990.

In their election manifesto the Conservatives said: “We will protect the green belt.”

Prime Minister David Cameron told CPRE: “Green belt land is extremely precious. Protecting the lungs around our cities is paramount for me.”

However, nearly half of all homes currently planned on green belt land are in London and the wider metropolitan area and the number of houses planned in the metropolitan area has more than tripled since August 2013.

But the shortage of new homes and the UK’s growing population has, CPRE says, led to “green belt policy gradually being weakened through loopholes in planning guidance”.

The organisation says councils are releasing greenbelt for new development via an ‘exceptional circumstances’ clause. CPRE cites three local authorities, Bradford, Durham and Northumberland, who have argued that economic growth means ‘exceptional’ change to the green belt is required.

Last month, Greg Clark, Secretary of State for Communities and Local Government, decided 1,500 new homes can be built between Gloucester and Cheltenham, one of the biggest green belt developments for a decade.

It followed changes to the government’s policy consultation to release small sites in the green belt for ‘starter homes’.

In July 2015, the Chancellor, George Osborne, vowed to take on ‘nimby’ councils and streamline planning laws to allow more homes to be built.

The CPRE wants the government to give councils powers to build on unused brownfield sites.

Paul Miner, CPRE campaign manager, said: “Councils are increasingly eroding the Green Belt to meet unrealistic and unsustainable housing targets. The Government is proposing to encourage further development in the Green Belt. Our Green Belt is invaluable in preventing urban sprawl and providing the countryside next door for 30 million people.

“We need stronger protection for the Green Belt, not just supportive words and empty promises. To build the affordable homes young people and families need, the Government should empower councils to prioritise the use of brownfield sites. Brownfield land is a self-renewing resource that can provide at least 1 million new homes.”

The CPRE cited research by consultancy Glenigan in June 2015 for BBC Radio 4’s File on 4 which, it said, saw an increase in the number of houses securing full planning approval in the greenbelt. In 2009/10, 2,258 homes were approved. By 2014/2015, it had risen to 11,977, a fivefold increase in five years.

A Department for Communities and Local Government spokesman said: “There are no plans or policy to relax the strong protections that prevent inappropriate development on the Green Belt.

“Ministers have repeatedly been clear that demand for housing alone will not justify changing Green Belt boundaries. Councils are already expected to prioritise development on brownfield sites with 90% of brownfield sites expected to have planning permission by the end of this parliament.

“It means that in 2014-15 just 0.02% of Green Belt was converted to residential use, and the Green Belt is actually 32,000 hectares bigger than it was in 1997.”