Security specialist Symantec is buying Gloucester-based messaging and web security specialist Messagelabs, in a deal worth $695m (£397m), subject to regulatory approval.
The money will be paid in £310m sterling and $154m in US dollars.
Symantec said in a statement that the deal would give it leadership in the software-as-a-service market and strengthen its lead in the messaging security field.
It described Messagelabs as the number one provider of online messaging security worldwide with more than 8 million users in more than 19,000 companies of all sizes.
Symantec chief executive John W Thompson said the deal would give the combined company "one of the strongest portfolios of cloud-based infrastructure services and a great foundation on which to grow”.
Messagelabs was founded in 1999 by UK brothers Ben and Jos White and has 512 staff in ten countries worldwide.