Shoosmiths is to merge with Scottish law firm Archibald Campbell & Harley.
The new business will be known in Scotland as ACH Shoosmiths, and will go live in the autumn subject to regulatory approval.
Shoosmiths, a top 40 national law firm with eight offices in England, has been prompted to move into Scotland as a result of client demand.
Chairman Andrew Tubbs said: "Clients have been telling us for some time that they wished we had a presence north of the border, and this is us responding to their wishes.
"In Archibald Campbell & Harley we've a perfect fit, in both business and cultural terms. We share clients and there's crossover in practice areas, particularly real estate, retail, litigation and recoveries for lenders.
"Opportunities will be created north and south of the border through cross-referral of both firms' existing clients, and as a result of Shoosmiths' ability to offer a true UK-wide service to national clients and prospects.
"This is a hugely positive step, and a tangible demonstration to clients, staff and others of our ambition. That our ninth office will be in Scotland is very exciting."
Based in Queen Street, Edinburgh, Archibald Campbell & Harley advises on wide range of matters, including banking and finance, commercial property, corporate, debt recovery, dispute resolution, employment, insolvency and restructuring, and private client work.
Founded more than 100 years' ago, it describes itself as 'one of Scotland's most focused and progressive legal practices, a mid-sized firm able to bring an uncommonly high standard of business intelligence to the affairs of our clients'.
All 65 partners and staff from Archibald Campbell & Harley staff will be retained in the merger.
Its chief executive Rod Macphail said: "We voted unanimously to merge with Shoosmiths, and Andrew Tubbs quite rightly describes this as a very good fit, with us having both client connections and strong similarities in the way we go about business.
"We have worked with Shoosmiths for a number of years with mutual clients so this is a logical progression of an existing successful relationship. There's a real buzz of excitement and anticipation here about the merger because it enables both firms to meet client demand for multi-jurisdictional legal advice."
Shoosmiths has offices in Basingstoke, Birmingham, Manchester, Milton Keynes, Northampton, Nottingham, Reading, and Southampton. The newest - Manchester - opened in 2009 with a financial services focus and six employees, and which is now close to full-service with more than 70 staff.