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Nearly half a million homes are set to be built on ex-Green Belt land - but just a fifth will be classed as “affordable”, research reveals today.

About 459,000 houses are planned for space that had been protected to prevent urban sprawl, according to a study by the Campaign to Protect Rural England.

Its annual State of the Green Belt report, published today, warns releasing such areas for housing is only supposed to happen under exceptional circumstances - and that “shrinking” the Green Belt is as harmful as building on it.

Today’s study claims that in August 2012, land released from the Green Belt was earmarked for 81,000 homes.

Yet that rocketed to 459,000 by June this year.

Evidence suggests land which had been set aside for protection is being freed-up to ease the chronic housing crisis gripping the country.

Yet the proportion of “affordable” homes is set to fall, experts warn.

Homes are deemed affordable if mortgage payments would be more than would be paid in rent on council housing, but below market levels.

Last year 72% of homes built on greenfield land within the Green Belt were unaffordable by the Government’s definition.

(Image: Library Picture/Western Mail Archive)

The CPRE says that of the 460,000 homes that are planned to be built on land that will be released from the Green Belt, the percentage of unaffordable homes will rise to 78%.

Policy director Tom Fyans said: “We are being sold a lie by many developers.

“As they sell off and gobble up the Green Belt to build low density, unaffordable housing, young families go on struggling to afford a place to live.

“The affordable housing crisis must be addressed with increasing urgency, while acknowledging that far from providing the solution, building on the Green Belt only serves to entrench the issue.

“The Government is failing in its commitment to protect the Green Belt – it is being eroded at an alarming rate.

“But it is essential, if the Green Belt is to fulfil its main purposes and provide 30 million of us with access to the benefits of the countryside, that the redevelopment of brownfield land is prioritised, and Green Belt protection strengthened.”

The CPRE says there is enough brownfield land in England for more than a million new homes.

But house builders prefer virgin sites because previously-developed land proves more expensive because of clean-up costs.

A Ministry of Housing, Communities and Local Government spokesman said: "We are clear that building the homes our country needs does not mean tearing up our countryside.

"Last year the number of new homes built was the highest in a decade, and only 0.02% of the Green Belt was developed for residential use.

"We are adding more certainty to the planning system and our new planning rulebook strengthens national protections for the Green Belt, and says that councils may only alter boundaries in exceptional circumstances once they have looked at all other options."