Large supermarkets and dairy processors “colluded to increase the prices of dairy products” between 2002 and 2003, the Office of Fair Trading has provisionally found.

The increase in prices led to an estimated cost to consumers of around £270m, the OFT said.

It has issued a statement of objections setting out its provisional findings to Asda, Morrisons, Safeway, Sainsbury's and Tesco, as well as Arla, Dairy Crest, Lactalis McLelland, The Cheese Company and Wiseman.

“This is a very serious case,” said OFT executive director Sean Williams. “We believe supermarkets have been colluding to put up the price of dairy products. This kind of collusion on price is a very serious breach of the law.”