The Office of Fair Trading conducted a series of dawn raids at offices of Barclays and Royal Bank of Scotland (RBS) as part of an investigation into price-fixing on loans to lawyers and accountancy firms.

The competition watchdog, which is already investigating price-fixing in several industries including supermarkets and tobacco, said that the inquiry was focused on a sub-section of the business banking market that lends to professional services groups.

Between them, RBS and Barclays have a 70 per cent share of the professional services banking market.

The investigation and raids followed a voluntary approach to the OFT by Barclays amid concerns that staff in its professional services banking group “had been approached from outside ... in an innapropriate manner”.

The approach, known as a leniency application, is significant because the party that first informs the regulator can secure immunity from any subsequent penalties. Virgin Atlantic avoided a fine for price-fixing on fuel surcharges after informing the OFT that it had been operating a cartel with British Airways, which later was fined £121 million.

The OFT said that its investigation was at an early stage and declined to comment on possible penalties. It has the power to fine businesses up to15 per cent of the annual turnover of the relevant business area. However, if the regulator believes that this amount is too small, it can impose a fine of up to 0.75 per cent of group turnover. RBS's global turnover was £29 billion last year, which could lead to a maximum fine of £218 million if it were found guilty.

The OFT said that its investigation had a narrow focus and was limited to Barclays and RBS, but news of the raid prompted speculation about which bank or business area might be next.

Although it is fighting a long-running legal battle with Britain's banks over what it believes to be unfair overdraft charges on current accounts, the OFT has rarely turned the spotlight on the banking sector.

John Pheasant, competition partner at the law firm Hogan & Hartson, said: “The investigation shows the OFT is prepared to take on the financial services industry. It would be surprising if this is the last visit it pays to UK banks.”

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