Two of Britain's biggest mortgage lenders have changed their lending criteria to force homebuyers in many booming hotspots in London and the South East to come up with a more substantial mortgage deposit.

As many as 1 in 4 postcodes in the region could be affected after NatWest and the Alliance and Leicester revealed that they have made the move amidst concerns over rapidly rising house prices, economic uncertainty and a perceived increase in the threat of negative equity.

Having traditionally allowed most homebuyers to put down as little as 5 percent of the property value as a deposit, both lenders are now requiring most buyers to stump up at least 10 percent and sometimes more. However, not all areas are affected and individuals are to have their cases looked at on an individual basis in those areas covered by the new policy.

A NatWest spokesman said: "In some areas within the M25 house prices have risen quite dramatically. We think it is prudent to reduce the maximum amount of overall lending relative to the amount of the property's (value) in some cases. The last thing we want to do is lend large amounts of money if there is a risk of negative equity."

The Alliance & Leicester says that it has been asking for 10 percent deposits on all properties bought in the whole of the South East since the autumn, following the combined bust-threat of a booming first half of last year and the ensuing global economic shakiness.

Some lenders and advisors have immediately signalled their unhappiness at the move, which they claim could lead to house prices falling in affected areas. Forcing first time buyers to save bigger deposits will put property beyond the means of even more potential owners, constricting the demand and thereby causing prices to fall.

Halifax, Nationwide, HSBC, Barclays and the Woolwich have all reacted by stating that they have no plans to raise the deposit required for mortgage lending, though the Halifax has promised to raise a complaint with the Council of Mortgage Lenders. However, a spokesman for the the CML, clearly aware of the restrictions on their power to act in such circumstances, said: "It is a matter for individual banks and building societies to establish their own position," said a spokeswoman from the council.