The move, which will give Berrymans offices in both Bristol and Cardiff, will consolidate the firm's relationship with insurance giant Axa Insurance.
Cartwrights is a longstanding Axa-panel firm and Berrymans is currently back on the panel following a brief suspension for failing an audit at the beginning of last year.
As well as having one of the largest insurance departments in the South West and southern Wales, Cartwrights is also known for leisure, transport, employment, property and company commercial work.
Berrymans has made it clear that it intends to rebuild the commercial side of its practice, but a source close to the firm claims that Berrymans is planning to merge with only the insurance practice at Cartwrights.
Since Berrymans merged with Lace Mawer in 1997, creating the UK's largest insurance practice, the commercial side of the firm has been sidelined.
Last July, the Manchester office's head of commercial litigation Tony Brook left to set up his own firm. And in October, Berrymans lost more than half of its Liverpool-based commercial department when six equity partners joined North West rival Brabner Holden Banks Wilson.
At the time, senior partner Paul Taylor said: "We're already looking at a number of options to increase and strengthen the commercial practice. We want to provide a commercial service which matches the national nature of the firm." (The Lawyer, 19 October 2000).
In 1998, Berrymans was in merger talks with Newcastle-based niche insurance firm Linsley & Mortimer. But Linsleys backed out and eight months later merged with Eversheds (The Lawyer, 17 May 2000).
Senior partner Paul Taylor declines to comment on the merger, but he says: "It is part of our strategy to develop geographically, particularly in the west of the country and in the North East."