Serious fraud against UK banks in the first half of 2008 was greater than in the whole of any previous year, according to KPMG.

And the credit crunch is likely to make matters worse, the report's authors say.

The Forensic Fraud Barometer (PDF), now in its 20th year, reports fraud cases of £100,000 or more which come to court in the UK.

More than £630m worth of alleged fraud in 128 cases was reported in the UK, some £350m of which was perpetrated against the financial services sector. The previous "high watermark" of fraud against banks was £200m in 1998.

A large proportion of the sum was from one case, still being tried, in which hackers attempted to steal £220m from the Sumitomo Matsui Banking Corporation. The next largest was a £70m fraud attempted against HSBC's Securities Division.

Organised criminal gangs are behind the heists, according to the report.

"Banks are working extremely hard to protect themselves and their customers from fraudulent activity, but the signs are that organised criminals and syndicates have been relentless in their efforts," said Hitesh Patel, a partner at KPMG Forensic.

The majority of the cases in the latest report predate the credit crunch, and Patel expects more cases to come to light as firms go over their figures more carefully.

"The signs are that we could end up seeing some substantial losses being suffered," he said.