The big four's expansion into non-food could give their opponents even more ammunition with which to attack them - according to a white paper from the KPMG/SPSL Retail Think Tank.

Retail Think Tank member Nick Bubb, of Pali International, warned: "This rapid strengthening into the non-food sector is one of the points that could leave supermarkets vulnerable to further accusations of their power and influence growing too great."

There was also a danger that the supermarkets' success in non-food could reduce the size of the whole retail market, said fellow Retail Think Tank member Paul Clarke, of Barclays Retail and Wholesale Sectors.

"It may be that as the high street is further pruned, more shoppers are pushed towards online retailers able to provide the only realistic challenge to supermarkets."