An explosion of "garden grabbing" in London is revealed today. A third of new homes are being built on land that used to be someone's garden.

The figure has almost doubled in a year, from one home in six, as developers concrete over the suburbs.

Critics warned the dash to cram flats and houses into spaces that were havens for people and wildlife will have irreversible consequences for quality of life in the crowded capital. It also raises the risk of flooding because gardens soak up rainwater.

The scale of the increase was disclosed in a parliamentary answer to Tory MP Greg Clark who has led a campaign against the garden grabbers.

From 1997 to 2005 the level crept up from 13 per cent of new homes being built on residential land to 16 per cent. But from 2005 to 2006 it shot up to 29 per cent - the biggest rise of any region.

Across the south-east region outside London, there was a similar increase but over a longer period.

Typically a developer will knock down a traditional family home and build a block of flats that occupies the original property's footprint and part of the garden. The result is less greenery, fewer trees and more imposing development.

Mr Clark accused the Government of covering up the figures until after the London elections because they were so controversial. His written question was originally due for answer on 24 April but was delayed until 6 May. Boris Johnson pledged to hold back the garden grabbing phenomenon, a flagship of his mayoral election campaign.

Tunbridge Wells MP Mr Clark said: "The figures for London are truly shocking. Gardens are vital to our capital."

He said ministers had driven the increase to meet artificial targets for building new homes on "brownfield land". Under current guidance, residential land is classed as brownfield, which is often wrongly assumed to mean derelict land or old industrial sites.

Mr Johnson told the Standard: "The Government has failed to protect open spaces in London. It is the duty of the Mayor to protect green spaces."

Housing Minister Iain Wright said: "The Conservatives are being disingenuous. These figures include the replacement of run-down houses and estates."

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