Tesco, Sainsbury's, Morrisons and Asda said they were examining a report from the Office of Fair Trading.

Five dairy processors were also accused of the collusion, which allegedly led to overcharging of an estimated £270m.

The OFT said the figures broke down as 3p extra for a pint of milk and 15p extra per quarter pound of butter.

The overcharging, between 2002 and 2003, also led to customers being allegedly overcharged 15p per half pound of cheese.

Breaches of the Competition Act of 1998, which prohibits agreements, practices and conduct that may have a damaging effect on competition in the UK, could incur hefty fines.

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