Mars and Reckitt Benckiser today joined Unilever and Procter & Gamble in admitting they had been dragged into the Office of Fair Trading probe into alleged price-fixing at supermarkets.

The four companies have all been contacted by the watchdog, which is seeking evidence about claims retailers and manufacturers fixed prices of a wide range of goods. It is thought the OFT has demanded emails and other documents.

Tesco, Asda, Sainsbury's and WmMorrison have been contacted by the OFT but deny any wrongdoing. Unilever and Procter & Gamble also insist they are innocent.

A spokesman for Procter & Gamble said the OFT visited its offices on Thursday.

"Consistent with our principles, we are fully co-operating with their inquiries," he said. "It is P&G's policy to fully abide by the spirit and letter of the law."

Unilever said the OFT contacted it on Friday.

The investigation has outraged retailers, who have questioned the tactics used by the OFT since the appointment of John Fingleton as chief executive in 2005.

The OFT last week apologised to Morrisons and paid £100,000 compensation over a press release that wrongly suggested the supermarkets group was guilty of fixing prices of dairy products.