The merger of BDO UK and PKF UK, announced in November, has been approved by partners of both firms.
The merger is expected to be finalised at the start of 2013.
The merged firm will operate under BDO International and is to employ 3.500 people and have estimated annual revenues of just under £400m.
PKF UK managing partner Martin Goodchild said: "The strategic decision to merge has now been made by the partners of both firms. Our primary focus as we move to completion will be to ensure both our clients and people benefit from the merger; they will be working with a resilient partnership leading from a position of strength and with a client service ethic at its heart."
In a recent interview with International Accounting Bulletin BDO UK managing partner Simon Michaels said: "Both PKF and BDO have an ambition to lead in the mid-market, domestically and internationally."
"Internationally, BDO is already there. Both firms have complementary strategic strengths and they make a really good fit. We are all of the opinion that mid-tier consolidation is long overdue."
"The merger will create the largest mid-tier audit practice in the market and also have significant advisory capabilities. We should be able to provide more competition to the Big Four," he said.
BDO UK and PKF UK in merger discussions