M&B confirmed this week that it was "actively considering" how an acquisition of Spirit could best be funded, with a private equity purchase of a minority stake in the business thought to be a likely route.
The proposal is among the conclusions of a strategic review of M&B's business, initiated after the company incurred a £274m loss on the closure of a proposed joint-venture property hedge fund with tycoon Robert Tchenguiz.
However, one City analyst does not believe that M&B could afford to buy Spirit - which would cost an estimated £700m to £800m.
Punch had been in talks about a 50:50 merger with M&B but pulled out a few weeks ago.
The analyst said that such a deal "would be good for Punch and bad for M&B".
He went on to suggest that M&B's plan to buy Spirit may be motivated by self-interest on the part of management.
He said: "The difference between the two deals is one where the management keep their jobs and one where they don't. I would put the chances of the deal going ahead at less than 50% at the moment."
M&B operates about 2,000 pubs in the UK, accounting for 3% of the country's total estate but 10% of its total pub turnover.
Punch bought Spirit's 1,832 pubs in 2006 for £2.7b, and the business now has 836 pubs after Punch transferred some of the managed properties to tenancies and leaseholds.
By Christopher Walton
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