Supermarket delivery drivers have been told to sign new contracts imposing a pay freeze or face the sack.

The row hinges on the scrapping of a pay-for-performance agreement, which has been in operation at Didcot since the Asda warehouse opened five years ago.

Under PFP, drivers - whose average pay can range from £25,000-£27,000 with overtime - are classified in three grades.

Drivers in the two upper grades were shocked when general manager Richard Fielding announced the replacement of the PFP system so that everyone will eventually be paid the same rate.

About 100 drivers employed at the Didcot depot have received a letter from Mr Fielding offering new contracts that they must sign by September 22 - or face dismissal three weeks' later.

There are no trade unions at Asda and drivers were reluctant to air their grievances in public for fear of losing their jobs.

But one driver told the Oxford Mail: "The effect is likely to be that many drivers will work more slowly in the run-up to Christmas - unless management changes its mind."

Mr Fielding was not available for comment yesterday, but Asda spokesman Madeleine Pollard said: "The change is in response to feedback from our colleagues themselves."