Pressure was mounting on Marks & Spencer boss Sir Stuart Rose today as his summer of discontent continued.

The firm is bringing forward its next trading update by a month prompting fears it has suffered another sharp slump in sales.

This raises fresh questions about the future of Sir Stuart weeks after a rocky annual meeting where shareholders expressed their anger over his new role as executive chairman.

The mood was not helped by a shock profits warning last month and the dismissal of food director Steve Esom, who was brought in from Waitrose less than a year ago but was forced to carry the can for the disappointing performance.

Things do not appear to have improved since esom was fired.

M&S insists Sir Stuart is the right man to lead the retailer back to form and has dismissed claims that the combined role of chairman and chief executive gives him too much power.

But the City is losing patience and the mood looks set to blacken with another dismal trading update.

The firm will release second-quarter figures on 2 October ahead of half-year numbers on 4 November. It will be the first time it has released quarterly figures since it scrapped the practice in 2006. City retail analyst Nick Bubb of Pali International said: "The fact that M&S is revealing its sales early reflects that life is tough out there. The figures are likely to be pretty grim."

M&S is losing out to low-price rivals Aldi and Lidl as shoppers feeling the squeeze try to make stretched household budgets go further.

Market research group Nielsen said the amount spent in M&S stores fell 2.1% in the four weeks to 9 August. Separate data from TNS showed sales were down 1.4% over the period.

Bubb said it looked "very bad" for M&S and implied like-for-like sales were down 8% once store openings and expansions had been stripped out. That is even worse than the 5.3% slump in the 13 weeks to 28 June.

M&S shares, which have lost more than half their value since January, fell sharply yesterday and were down 1p to 254¾p today.

"The tide seems to be against M&S at the moment," said Barclays Wealth analyst William Hobbs. "M&S food is likely to be struggling severely."

While M&S is struggling, business is booming at its cheaper rivals. Nielsen said sales at Iceland surged 14.8% in the four weeks to 9 August while TNS showed sales at Lidl grew by 8.4% and at Aldi by 13.6%.