The Office of Fair Trading (OFT) has said it is to look into fresh allegations of collusion between supermarkets and suppliers.
The competition watchdog has demanded information from the big four supermarkets and some suppliers in order to investigate the charge that they shared information on the price of groceries and health and beauty products.
In a separate announcement, the OFT said it would investigate alleged unlawful practices in relation to the retail price for tobacco products.
The new allegations coincide with the end of the Competition Commission's two-year probe into the grocery market, throughout which it has taken the view that open competition between supermarkets gives the consumer adequate choice of supply, and that small shops can compete without protection from supermarkets' tactics.
Sainsbury's and Tesco said they had been asked for information by the OFT and were co-operating. Unilever, Procter & Gamble, Reckitt Benckiser and Mars confirmed that they had received letters from the OFT requesting information.
Stores under scrutiny for the tobacco price probe include Tesco, Asda, Sainsbury's, Morrisons, and the Co-op Group, as well as T&S stores, now part of Tesco, and TM Retail, which has evolved into the Martin McColl chain.
A spokeswoman for the Co-operative Group said: "This is an ongoing industry-wide inquiry by the OFT which has itself said it should not be assumed that parties involved have broken the law. We have co-operated with the OFT on this for the past three years, and will continue to do so."
Tobacco product manufacturers Gallaher and Imperial Tobacco were also approached. An Imperial spokesman said: "We take compliance with competition law very seriously and reject any suggestion that Imperial has acted in any way contrary to the interests of consumers."
The tobacco groups and retailers have a month to respond to the charges.