Asda approached the Office of Fair Trading (OFT) with e-mails related to a separate investigation by the Competition Commissioner, according to the Financial Times.
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Tesco, which was in a price war with Asda at the time, also gave e-mails to the commission.
Earlier this year the OFT raided Tesco, Sainsbury's, Morrison's, Asda and Procter & Gamble. It also asked for information from grocery makers including Nestle, Unilever, Cadbury, Mars, Coca-Cola and GlaxoSmithKline, the FT said.
The full OFT statement reads: "The OFT is conducting an ongoing investigation into potential breaches of competition law by a number of retailers and suppliers, across a range of product areas. In the interests of transparency, the OFT has written to all the parties providing an update on the investigation. It is inappropriate to give any further detail at this stage."