Northern Foods revealed plans to mothball the site, which supplies M&S with ready meals, earlier this month after it was unable to agree a new supply contract with the retailer, leaving 730 workers facing redundancy.
More than 100 staff from the factory, as well as leaders from the GMB union, met with three M&S directors after their protest cut off a corner of London's busy Marble Arch.
“M&S is making money, Northern Foods is making money, they don't have to close the factory,” Paul Kenny, GMB general secretary told The Times. “It has made money consistently up until last year. They just don't understand.”
A spokeswoman for M&S said: “We worked really hard with Northern Foods to reach an agreement and we just didn't get there.”
When news of the mothballing broke two weeks ago, The Grocer revealed Northern Foods' refusal to agree to M&S's new terms had been applauded by suppliers enraged by the retailer's aggressive attitude and its controversial new food and drink scheme Project Genesis.