More than 150 politicians have signed a letter penned by Labour MP Siobhain McDonagh, which accuses Asda of trying to push through “unscrupulous contract changes” that would leave thousands of workers worse off.

The letter will be sent to Asda boss Roger Burnley, dealing a fresh blow to the supermarket chain just a week after its proposed merger with Sainsbury’s was blocked by competition authorities.

Last month Asda said it was consulting with staff over proposals to simplify its terms for hourly paid workers in its stores. As part of the plans, it aims to raise the basic rate of pay to £9 per hour for all of its workers.