Law firm Withy King, which has an office in Swindon, is to merge with London firm Royds.
The tie-up, which will take effect from September 1, will create a £33m turnover business employing around 450 people.
Withy King also has offices in Swindon, Marlborough, Oxford and Thame and a small base in London, while Royds operates from a single office in the City.
Royds is known for its retail property, commercial litigation, commercial, banking, employment, family and private client practices.
It also has a strong international client base and is a founding member of the international association Interleges.
Withy King is ranked 45th in The Sunday Times 100 Best Companies to Work For in 2016 and entered The Lawyer Top 100 for the first time in 2015. The firm merged with Swindon-based single-office practice Lemon & Co two years ago – adding around 40 staff, including three full equity partners, and a turnover of £2.4m to the firm.
Withy King also acquired law firm Whetter Duckworth Fowler based in Wheatley, Oxfordshire, in December 2013. 
Withy King managing partner Graham Street, who will stay in his role after the merger, said: “This is an exciting opportunity which will bring significant benefits for both firms and their clients.
“Withy King and Royds have complementary strengths and share a similar vision and culture, making them a really good fit.
“We believe the merger will provide sustainable growth opportunities for our commercial and private client practices in current markets as well as in new and larger markets, both in the UK and internationally.
“For our clients, it means more choice and access to an even wider range of services and legal expertise.”
Royds managing artner Stewart Wilkinson added: “We are very much looking forward to merging with Withy King which is recognised as an ambitious and dynamic practice with an excellent track record.
“There are many synergies between our firms and this merger presents us with excellent opportunities to grow and develop across all areas of our practice. In particular it will enable us to extend the range of specialist services we offer to our clients who are based both here and abroad.”
An announcement on the name of the merged firm will be made at a later date, Withy King said.
