Other companies including Asda and Sainsbury’s, along with Scotland’s Robert Wiseman Dairies and cheesemaker McLelland before its takeover by Lactalis, admitted in 2007 that they were part of a price-fixing group that cost shoppers an extra £270m.

The firms settled with the OFT in December 2007 and between them paid up fines totalling £116m. The industry had claimed prices were set over two years between 2002 and 2003 to help farmers after the outbreak of foot-and-mouth disease and that they made no extra profit as a result of the price fixing.

Wiseman and five other smaller Scottish dairies were last year cleared in a separate OFT investigation over price fixing in Scotland, following a complaint by rival Arla.

The OFT not only made it clear it was still pursuing Tesco and Morrisons four-and-a-half years after launching the inquiry, but said it had “additional evidence” in the form of a “supplementary statement of objections” in support of its earlier findings.

“The parties that are continuing to contest the OFT’s provisional findings are Morrisons and Tesco,” the watchdog said. “They now have an opportunity to make written and oral representations in the response.”

Tesco executive Lucy Neville-Rolfe said: “We will look carefully at any new evidence the OFT sends to us. We will however continue to defend our position strongly.”

Morrisons also said it would contest the provisional findings and that it would wait to read the document in detail. “It remains our firm belief that there are no reasonable grounds for the OFT’s allegations against us.”