Revealed: The 100 top brands at the centre of OFT price-fixing probe

GlaxoSmithKline, Coca-Cola, Procter & Gamble, Unilever and Kimberly-Clark confirmed they had received visits or letters from the Office of Fair Trading.

Nestle, makers of Nescafe coffee and Maggi instant noodles, and Britvic, which owns Robinsons, Pepsi and Tango, have also been spoken to by the OFT.

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The revelation follows last week's raids on Britain's four biggest supermarkets - Tesco, Asda, Sainsbury's and Morrisons - by the consumer watchdog.

Hundreds of emails between the supermarket chains and suppliers of groceries, detergents and health and beauty products were seized.

According to the Daily Telegraph, the OFT inquiry focuses on about 100 well-known brands including Anadin Extra, Cadbury Highlights, including PG Tips, Andrex, Aquafresh toothpaste and Warburtons bread.

An OFT spokesman said the body would neither confirm nor deny anything about an ongoing investigation.

It was also reported last night that suppliers and retailers had been given until June to hand over pricing information wanted by investigators.

GlaxoSmithKline confirmed that the body had approached them about its toothpaste product Aquafresh.

Phil Thomson, spokesman for GlaxoSmithKline, said: "I can confirm that we have had a letter from the OFT, which we received last week.

"We will be responding to them in due course. I can confirm that it relates to Aquafresh."

Procter & Gamble said in a statement last night: "We can confirm that Procter & Gamble were visited by the OFT on Thursday. Consistent with our principles, we are fully co-operating with their inquiries.

"We do not have any additional information to share at this stage, save to say that it is Procter & Gamble's policy to fully abide by the spirit and letter of the law."

Other brands reported to be on the OFT list include Colgate Total toothpaste, Dove Moisturiser, and biscuits manufactured by Northern Foods, one of the largest food producers and a major supplier to Marks & Spencer.

A spokesman for Unilver, which makes Dove, said: "We were asked on Thursday by letter to supply information to the OFT. That is what we are currently working on."

A spokeswoman for Coca-Cola Enterprises said: "I can confirm that we have been contacted by the OFT for information and that we would expect to meet them in due course."

Kimberly-Clark, the US owner of toilet paper manufacturer Andrex, said in a statement: "We have received a letter from the OFT and will assist with this investigation in any way we can."

John Bridgeman, a former director general of the OFT, said: "Supermarkets account for a huge amount of consumer expenditure. It is a £100 billion sector of the economy.

"There is no smoke without fire. The OFT is not going to go big on this unless it has something to go on."

Last week it emerged that 11 major retailers and two tobacco manufacturers were facing an OFT investigation over allegations they illegally fixed the price of cigarettes.

Tobacco giants Imperial Tobacco and Gallaher were named alongside chains including Tesco, Sainsbury's and Asda in a 'statement of objections' issued by the OFT.

The watchdog has accused the groups of anti-competitive pricing in a period spanning 2000 to 2003.

The OFT has been increasingly forceful in protecting consumers in recent months and has launched an assault on anti-trust activity. It is also investigating alleged price-fixing in the construction industry.

All the supermarkets and food manufacturers involved deny any wrongdoing and reportedly see the latest investigation as a "fishing expedition".