The retail giant was down around 3%, after shedding almost 3% yesterday, as new market share data poured doubt on its clothing turnaround and City analysts downgraded its prospects.
Chief executive Marc Bolland is attempting to halt eight consecutive quarters of falling general merchandise sales - which span clothing and non-food - with an advertising campaign featuring Dame Helen Mirren and Olympic boxer Nicola Adams.
But figures from researcher Kantar Worldpanel showed M&S's share of the womenswear market fell further in the 24 weeks to the end of August.
The retailer lost 0.6 percentage points of market share, compared with 0.7 percentage points in the prior month, the Financial Times reported.
The data covers the early days of its new autumn/winter ranges, which began arriving in stores from July but were only formally launched at the start of September.
Meanwhile, analysts at Deutsche Bank and Credit Suisse cut their general merchandise forecasts to negative for the three months to the end of September.
After the chain disappointed with a 1.6% fall in like-for-like general merchandise sales in the 13 weeks to the end of June, Deutsche analysts now expect a 2% second-quarter decline, compared with a previous forecast for flat sales.
Credit Suisse analysts piled on the pressure by predicting a 1.5% general merchandise sales fall in the second quarter.
The print, billboard and online campaign featuring a dozen "leading ladies" was snapped by renowned portrait photographer Annie Leibovitz.
It was unveiled in August and features the women at locations including a boat on the River Thames and a country house in Berkshire, wearing dinner dresses, leather jackets, faux furs and stiletto heels from the ranges.
Credit Suisse analysts said while the long-awaited autumn/winter ranges show "some signs of improvement", they will not be enough to transform underlying sales or margins.
And while July's heatwave should have helped the chain, they said the weather boost will have diminished in August and September, resulting in "disappointing trading".
Deutsche Bank analysts added the clothing market has been "challenging", with retailers having to resort to heavy discounting - highlighting the chain's summer sale starting a week early.
They said: "Despite 'new' ranges arriving in store from July we doubt sufficient evidence will be given on the performance of these to persuade us or the market that the turnaround has definitely begun."
Deutsche Bank cut its pre-tax profit expectations for the first half to the end of September by about £15 million to £281 million.
Data published today by the British Retail Consortium (BRC) and KPMG showed shoppers waiting for colder weather before restocking winter wardrobes.
They said like-for-like sales growth of 0.7% on a year ago made September the weakest month of the year to date.
Unseasonably warm weather stifled sales of autumn and winter clothing collections, with cautious shoppers happy to leave purchases until later on.
Analysts at Societe Generale believe a "compelling transformation" is currently running below the surface of M&S, and reckon trading in its crucial third quarter to the end of December could show better clothing sales.
"Ultimately, we think it is capable of joining a select group of 'retail defensives', with a permanent share buyback strategy in years to come," they said in a note last week.
M&S reports first-half results on November 5.