Royal Bank of Scotland and Barclays London offices have been raided by the Office of Fair Trading, seizing documents and phone records, in the latest example of the competition watchdog’s hard-line approach to possible price-fixing.

The documents were taken as part of an investigation in to alleged anti-competitive behaviour over the pricing of loans to professional services firms, people involved in the probe said.

The probe was said to have been triggered by Barclays and comes amid high-profile investigations into price-fixing allegations involving supermarkets, consumer goods companies and tobacco groups. The OFT has confirmed they are in the early stages of the investigation and have a “narrow focus” into allegations of anti-competitive conduct in the financial sector.

RBS confirmed that its office at 280 Bishopgate had been raided and that it was fully co-operating with the authorities. Neither bank would comment, however, on the scale or scope of the material seized by the watchdog.

Barclays claim that members of its professional services team had been approached from outside the bank “in a manner which we regard as inappropriate”. The bank reported the incident on March 17, applying for leniency under rules that allow whistle-blowing companies to escape fines potentially amounting to as much of a tenth of global sales.

Barclays said: “The [OFT] investigation is operating within the confines of the professional services banking area and we believe that, if there is any issue, it starts and stops there.”

The OFT, in the past, used a more aggressive approach to tackling price-fixing, imposing heavier fines and using tough legal powers that allow it to target executives with the threat of up to five years in jail.

Only last year British Airways were fined a record £121.5m by the OFT for fixing the prices of passenger fuel surcharges. BA’s co-conspirator, Virgin Atlantic, applied for leniency and escaped a fine.

Banks are under increasing pressure from competition authorities in areas such as overdraft fees. The competition Commission is expected to publish a report later this week that will accuse leading banks of profiteering in the provision of loan insurance.