The deal will make the French insurance company, which also has operations in Asia and the rest of Europe, one of the biggest players in Switzerland.
The sale of Winterthur has been expected for the past two years, since its owners Credit Suisse decided to concentrate on banking.
Sullivan & Cromwell, led by Paris managing partner William Torchiana, is advising Axa on the corporate aspects of the deal. European counsel Juan Rodriguez advised on antitrust issues. Bär & Karrer, led by Rolf Watter, was Swiss counsel to Axa.
Lenz & Staehelin, led by partner Rudolf Tschaeni, is acting for Credit Suisse. Dewey Ballantine acted as US counsel to Winterthur.
Axa is financing the acquisition through a mixture of a rights issue and debt, which will include a hybrid bond. Hybrid bonds combine features of both debt and equity in one instrument.
The prospectus has already gone out for the €4.1bn (£2.8bn) rights issue, which is expected to take place in the next few weeks.
The debt package is expected to be around E4.8bn (£3.28bn) and, as well as the hybrid bond, will include senior and subordinated bond issues. The hybrid bond is expected to contain both sterling and euro denominated tranches.
Linklaters’ Paris office is advising Axa on the financing package, led by partner Gilles Endréo along with partner Severin Robillard.
It is understood that Allen & Overy will act for the banks.