Plexus Law is to merge with insurance dispute resolution firm Greenwoods in a deal that will create a £90m defendant insurance litigation business.

Plexus is part of the Parabis Group, which last year sold a majority stake in its business to private equity house Duke Street, handing it a war chest of around £50m to fund ambitious expansion plans.

Parabis, which also owns insurance litigation firm Cogent Law, last year took in total turnover of £108m. The firm claims the Plexus-Greenwoods merger will create a £90m defendant insurance litigation business, with total turnover for the Parabis Law LLP standing at £150m, a figure which would place it just outside the UK top 20.

Heads of terms for the tie-up have been signed, with the deal expected to complete in mid-May, creating a combined firm with more than 1,200 staff. Greenwoods' brand is set to remain alongside Plexus as part of the deal.

Greenwoods focuses on catastrophic loss claims, pure insurance coverage, major property damage and construction liability claims. Legal Week sister title Post reported in 2011 that the firm expected to take in turnover of £20m for the calendar year.

Greenwoods partner Malcolm Henke will become senior partner in the merged firm and join the executive management team at Plexus alongside fellow Greenwoods partner Richard Houseago. Greenwoods senior partner Paul Parsons will step back from day-to-day management of the firm, but will remain involved in the integration process.

Parabis chief exec and Plexus senior partner Tim Oliver (pictured) said: "This is a great deal for both firms. We have been respected competitors for many years. Together we have greater strength in depth and capability the better to serve our clients in this rapidly developing market.

"Clients want to work with firms that offer them a full range of practice capabilities and geographic reach. By joining forces with Greenwoods, and on the back of our merger announced in 2011 with Everatt & Co, we have high-end specialist expertise and strength in depth capable of running the biggest and most complex of cases."

Last year, Parabis became one of the first major law firms to become an alternative business structure, permitting Duke Street to take a majority stake in its business. The investment valued Parabis at between £150m and £200m, with the financial backing understood to be earmarked for a programme of growth and acquisitions.