The Office of Fair Trading (OFT) confirmed it had launched an investigation into "alleged anti-competitive conduct" by Barclays and Royal Bank of Scotland and raided two premises.

The watchdog said the probe had a "narrow focus", relating to the provision of loan products to professional services firms.

It added: "The OFT's investigation is at an early stage. The OFT will not be in a position to conclude whether the law has in fact been infringed until it has completed its investigation and assessed the available evidence."

Barclays said it contacted the OFT on March 17 after it became aware that certain members of the professional services team in its commercial banking operation had "been approached from outside Barclays in a manner which we regarded as inappropriate".

These concerns were sparked by alleged approaches from RBS to Barclays over sharing information such as the cost of loans to legal and accountancy firms, according to a Sunday Mirror report.

Barclays has applied for leniency, meaning it could be spared a fine if the OFT uncovers any wrongdoing. Firms can be fined up to 10% of their turnover under the Competition Act.

A spokesman said: "The OFT started an investigation in which Barclays is participating.

"The 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."

An RBS spokesman said: "As the OFT has stated, they are in the early stages of an investigation. We are, as always, co-operating fully with the regulatory authorities and it would therefore be inappropriate to comment further."