Royal & SunAlliance is to acquire loss adjuster Cunningham Ellis & Buckle's domestic claims handling service, Select Service, for an undisclosed sum.
The two companies have been working towards the deal since RSA first showed interest in buying the service in 1997.
Expected to be effective from December 1, the deal means that RSA would have a network of some 270 field advisers looking after around 3.5m homeowners.
About 100 field advisers from CEB will switch to RSA which will also take over the Select Service central office in Morley, Leeds and its 50-strong support staff.
The two companies joined forces in 1997 when CEB won a five-year contract to handle the insurer's household claims.
From December, the loss adjuster will have an exclusive three-year contract to handle RSA's domestic claims that cannot be handled by the Select Service.
RSA claims director David Neave said: "Having worked in partnership for the last two years, we decided the time was right for us to take complete ownership of the claims adviser network."
CEB has a network of 50 offices which offers a combination of loss adjusting and high volume low ticket claims handling.
It is the only loss adjuster of RSA's subsidence panel which also includes a number of engineering firms.