The competition watchdog said in a letter that there were “reasonable grounds to suspect” that pricing data was passed between supermarkets via suppliers, the Financial Times reported.
Earlier this year the Office of Fair Trading raided the offices of Asda, Morrisons, Sainsbury’s and Tesco, as well as FMCG giant Procter & Gamble.
Information was also requested from suppliers including Unilever, Mars and Coca-Cola Enterprises.
No company has been accused of breaking the law.
The inquiry encompasses products ranging from tea to toothpaste and was originally sparked by Asda, which alerted the Office of Fair Trading when it discovered information during a trawl of emails.
If it is decided that a cartel has been operating, companies involved can be fined up to 10 per cent of their turnover.