Yorkshire-based Bradford and Bingley has already declared losses of £15m resulting from organised mortgage fraud and experts warn that other firms could also be hit.

The Association of Chief Police Officers recently claimed total mortgage fraud in Britain amounted to £700m and was driven by "the current low risk of detection and high profit opportunities".

Organised frauds can involve corrupt professionals who conspire to over-value properties to obtain extra mortgage funding and often target multiple properties and new-build schemes.

But it also includes individuals who lie about their earnings to borrow larger sums of money, typically on self-certification mortgages.

A forensic accounting expert in Yorkshire said: "Mortgage frauds follow the housing cycle. Both types of fraud only come to light when property prices start to fall and properties go into arrears and you get repossessions. It is a problem across the industry. We have some of the largest mortgage lenders in the North. It's going to be a problem for those lenders. How much have they lost?"

The concerns have come as Bank of England figures reveal the number of new loans approved for house purchases fell to the lowest level since current records started. The number of mortgages fell from 41,000 in May to 36,000 in June.

A new Government-commissioned report published yesterday outlined ways to revive the ailing mortgage market and warned it would be at least two years before conditions returned to normal. The former head of HBOS, Sir James Crosby, said it was inevitable problems in the mortgage market would hit house prices.

While negative equity is well publicised for home owners, accountants believe the fall in house prices is revealing a rising number of discrepancies between valuations and actual prices.

It is understood new-build complexes are particularly at risk of organised mortgage fraud, and cities such as Leeds are ripe for exploitation because of the explosion in the number of new-build flats in recent years.

This spring, one investor negotiated a big discount with a developer to buy some flats, the Yorkshire Post has been told. The investor collected a five per cent deposit from buyers and then disappeared.

Andrew Frankish, of Rotherham-based brokers Mortgage Talk, said: "During a property boom there is an opportunity for organised fraudsters. That is one of the reasons why lending criteria is far tighter."

Frauds could be executed through investment clubs, Mr Frankish warned, and could be difficult to spot.

Property market professionals are seeing what they believe could be the start of a trend.

Andrew Wells, partner in charge of the Leeds auction house Allsop, said: "As long as prices continue to rise and rise and rise, the fraud gets covered and is not seen. But when the tide goes out, you will see what's on the beach.

"It is quite early days. We have only had the downturn for six months. I think if we transported ourselves to July 2009, I would not be surprised if there was much more of it about."

A Bradford and Bingley spokeswomen said: "We take mortgage fraud extremely seriously.

"To help combat this problem we have a number of systems and controls in place to identify suspicious applications as early as possible."

The Financial Services Authority said incidences of fraud had become widespread in recent years as a result of "easy credit conditions and streamlined application processes".

It is already attempting to tackle organised mortgage fraud, including visiting 200 mortgage intermediaries to assess their financial systems and controls and improving its work with law enforcement partners.