Bond Pearce will become the largest firm in the South West when it takes over the Southampton firm Hepherd Winstanley & Pugh on 1 May.
The 40 partner firm is currently the fourth largest in the region by fee earner size but, with 210 fee earners in all, it will leapfrog Bevan Ashford, Burges Salmon and Osborne Clarke when Hepherds' 14 partners come on board.
The move has pre-empted the anticipated merger between Bristol-based Wansbroughs Willey Hargrave, London's Beachcroft Stanleys and Manchester's Vaudreys.
But Bond Pearce senior partner Nigel Theyer denied the rival firms' talks were a catalyst for his firm's move - which he insisted was "definitely a merger and not a takeover".
He said: "Merger has been the strategy for a while, and this has been the culmination of discussions over the last few months." He added that the two firms were "a very good fit" in terms of their structure and practice areas.
Hepherds has a commercial practice, boasting retail, education and pharmaceutical client-bases, while Bond Pearce has strengths in the commercial sector, which includes energy, insurance, shipping, banking and insolvency.
Bond Pearce already has offices in Plymouth, Exeter and Southampton and it is envisaged that the two firms' Southampton offices will be brought under one roof within the next 18 months.
Hepherds' current managing partner Andrew Chalkley is to join Bond Pearce's management board.
Bevan Ashford chief executive Nick Jarrett-Kerr commented that it was "an interesting merger."
He said: "We don't regard their dominance in Southampton as an issue for the firm - we wish them well."
However he said that the firm was not itself seeking a merger.