Marks & Spencer's admission of interest in a potential bid for Sainsbury's could press the retailer's private equity suitors into tabling a firm offer for the company, a retail analyst has said.

M&S said it had considered a bid for Sainsbury's but decided not to make an offer, dampening speculation after chief executive Stuart Rose told a conference that the idea of a bid was "interesting" to him.

A private equity trio of Kohlberg Kravis Roberts, CVC and Blackstone said they were considering a bid for Sainsbury's nearly four weeks ago but no definite offer - expected about the 550p a share mark - has been forthcoming. Shares in Sains-bury's were up more than 2.5 per cent this week.

Nick Bubb, of general and food retail research at analyst Pali International, said: "St-uart Rose has said things before and got into trouble for it, but it shows that M&S has been looking at it seriously - perhaps more seriously than some of us thought.

Under take-over rules, M&S is now barred from making a bid for Sainsbury's in the next six months.