An MP has warned retailers that she is “coming after them” on pay as she delivered a petition with 90,000 signatures to Marks & Spencer bosses.

Labour MP Siobhain McDonagh declared that she will be taking action against the “huge number of retailers” that have been “involved in reducing people’s take-home pay” in order to bring in the national living wage.

As she handed over a petition with more than 90,000 signatures to bosses at Marks & Spencer’s flagship store on Oxford Street earlier today, she told Retail Week: “I can assure all those [retailers] that, if I can, I am coming after them.”

MP Siobhain McDonagh warns retailers 'I am coming after them'

McDonagh was joined at the store by a delegation including MPs John Spellar and Carolyn Harris among other campaigners.

She said: “A pay rise should be a pay rise. Changing terms and conditions so your pay goes down is against the spirit of the living wage.”

When asked why she was targeting Marks and Spencer in particular, McDonagh said: “Members of staff came to us who have been working for the company for more than 12 years, and a number of them are going to get £1,000 or £2,000 less a year.”

Despite the retailer’s 70,000 employees getting a pay rise across the board following the introduction of the living wage in April, McDonagh said: “It is not fair that some staff will receive £40 a week less as a result of the changes.”

She added: “Marks & Spencer has had its problems but it is a thriving retailer. It is in their best interests to be good to their best and most longstanding members of staff.”

The MP said she would like the Government to “close the loopholes that allow employers to avoid increasing people’s take home pay”.

The petition, created on on, challenges proposed changes to Marks & Spencer’s pay scheme including the scrapping of premium pay on Sundays, reducing payments for bank holiday work and changes to its pension scheme.

Other retailers, including Tesco, B&Q and Morrisons, have also introduced changes to their pay arrangements following the introduction of the living wage.

B&Q came under fire for making changes to its weekend pay and bonus scheme after it increased its minimum basic pay to £7.66 an hour in April, more than Chancellor George Osborne’s national living wage of £7.20 per hour.

Like Marks & Spencer, the Kingfisher-owned retailer said it will compensate staff who have been hit by its new pay structure for two years.

However, today McDonagh said: “We question whether Marks & Spencer are really going to compensate people for two years. The figures we’ve seen on the consultation document always fall short of what people actually earn.”

A Marks & Spencer spokesman said: “We believe our proposed new approach to pay and pensions would reward our people in a fair and consistent way, simplify and modernise our business and help us attract and retain the best talent so we can continue to provide great service for our customers.

”The proposals, which are subject to consultation with our employees, include one of highest pay rates and one of the best benefit packages in UK retail.”