Six partners will transfer to the volume defendant firm, including Squire & Co senior partner Nick Squire.
Plexus Law senior partner Tim Oliver said the move was part of a continuing plan to grow the firm through mergers and acquisitions.
Since it was launched in 2000 the firm has increased its staff to more than 1,500.
Squire approached Plexus about a possible tie-up because of plans to “wind down a little”, Oliver said.
“We’ve had some very fast growth and it’s all in insurance law. We’re not predatory, but we’re very keen to come together with other firms in a collaborative way.”
Oliver said the firm was building up its presence in preparation for the implementation of the Legal Services Act.