The UK's Office of Fair Trading has expanded its probe into alleged price-fixing between supermarkets and their suppliers to include various food giants, including Unilever and Procter & Gamble.

While the OFT refused to confirm that its price-fixing probe had expanded, both P&G and Unilever told just-food today (28 April) that they had been contacted by the UK regulator.

Procter & Gamble confirmed that it was visited by OFT officials last Thursday, when the competition watchdog also visited the headquarters of Tesco, Sainsbury's, Morrisons and Asda to gather information on the prices of groceries and other goods.

"Consistent with our principles we are fully cooperating with their inquiries. We do not have any additional information to share at this stage save to say that it is P&G's policy to fully abide by the spirit and letter of the law," a spokesperson for the Pringles maker said.

However, the consumer goods giant declined to reveal which products were included in the investigation with the probe ongoing.

The OFT also wrote to Unilever to request information, a spokesperson for the company said.

"I can confirm that they contacted us on Friday with a request for information and that we are in the process of responding," Unilever told just-food.

According to UK reports, the OFT is examining correspondence between supermarkets and 20 other food and non-food companies, including the likes of Mars.

Mars was unavailable for comment as just-food went to press, but it has been reported that the confectionery giant received a letter from the OFT requesting information late last week.

The high-profile investigation is likely to do little to ease tensions between competition regulators and retailers.

Last week the OFT was forced to issue an apology and pay Morrisons GBP100,000 (US$156,144) in damages in an out-of-court settlement over mistakes in a statement about alleged dairy price-fixing.

This week, the Competition Commission will formally publish the findings of its latest investigation into the grocery sector.

It is expected that the Competition Commission will call for the establishment of a "supply ombudsman" to oversee retailers' relationships with their suppliers.