Slaughter and May is acting for longstanding client Punch Taverns in a merger bid that would create the UK's largest pub group.
Slaughter and May's team is being led by partners David Johnson and Kathryn Davis.
Johnson said: "We started acting for Punch when it was first formed and followed the company through its flotation (in 2002)."
He added that merger proceedings were at a "very preliminary stage".
Punch tabled an offer last week (Monday 4 January) for Mitchells & Butlers (M&B), which runs the Harvester and All Bar One Chains.
Allen & Overy is representing M&B in what looks set to be a lengthy battle for control of the company. The magic circle firm has represented the company since it emerged from the 6C demerger in 2003.
Any deal would need the backing of property tycoon Robert Tchenguiz, who owns a 23 per cent stake in M&B through his R20 investment vehicle. The entrepreneur is historically a client of Kirkland & Ellis and partners Graham White and Raymond McKeeve, who refused to comment on the merger.
Punch's offer would see shareholders of each company receive half the shares in the new group, with M&B shareholders also benefiting from a £175m sweetener. Bidders have been circling the M&B since it announced a £391m loss from a failed property transaction.