Shortly after midday, the FTSE 100 was 37.55 points higher at 6,256.81.
M&S shares sank 8% after the retailer warned of a hit to short-term profits as it attempts to revive its clothing business.
New chief executive Steve Rowe says he intends to lower clothing prices and reduce the number of sales promotions.
But bank shares helped to bolster the wider market, with RBS up 3.6% and HSBC 3.4% higher.
Royal Mail shares rose 1% to 526.5p after regulator Ofcom said it would not impose any new price controls on the company.
Ofcom said it had decided against new controls because of the declining market for letters and increasing competition for parcel deliveries.
Electrical goods and mobile phone retailer Dixons Carphone rose 0.5% as it pushed up its full-year profit forecast after enjoying strong sales. It now expects profits of between Â£445m and Â£450m.
On the currency markets, the pound rose 0.3% against the dollar to $1.4685, and was also 0.3% higher against the euro at â‚¬1.3174.
(Noon): Banking shares helped to lift the market, but shares in M&S slid after the retailer said its turnaround plan was set to hit profits.