Asda has come under mounting pressure from MPs to drop its controversial new pay proposals for shopfloor workers.

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.

But according to City AM, McDonagh’s letter accuses Asda of using the new rates of pay as a “smokescreen” for scrapping unpaid breaks and shortening night shifts.

She has called on the business to “ensure that no Asda employee receives a pay cut” following the changes, after the grocer admitted 3,000 staff could be left worse off.

However, Asda insisted 95% of workers would be better off following the proposed changes, and said it would make transitional payments to those who would receive less pay.

McDonagh claimed 2,700 employees will lose as much as £500 per year, while another 300 would miss out on even more if the contract changes go through.

An Asda spokeswoman said: “We are currently consulting with our colleagues and their representatives over a proposal to invest in an increased rate of pay and changes to terms and conditions, which would enable us to deliver better service to our customers in an intensely competitive marketplace and would make 95% of our colleagues financially better off.

“This consultation is ongoing and we will always have conversations about change with our colleagues first.”