M&S rose 1.1% after its latest trading update, which said underlying sales of clothing and homewares fell 0.4% in the past quarter.
But the retailer said it was on track to meet its planned increase in profit margins.
The FTSE 100 index was down 3.50 points at 6,532.18.
The market had fallen 0.8% on Monday as investors reacted to Greece's rejection of the terms of an international bailout.
Shares in Asos fell 0.8% despite the online fashion retailer saying its full-year sales growth was set to be at the "higher end" of its 15-20% projections.
Asos said retail sales in the four months to 30 June grew by 20%, with UK sales up 27% while international sales climbed 16%.
On the currency markets, the pound slipped 0.04% against the euro to â‚¬1.4110 and fell 0.3% against the dollar to $1.5553.
(Open): The London market opened flat, but shares in Marks and Spencer opened higher despite the retailer reporting a dip in non-food sales.