Oxfordshire anti-virus software firm Sophos has sold a majority holding to a private equity company in a deal valuing the business at £547m, it was announced today.

The investment by Apax Partners will net the founders of the business, Jan Hruska and Peter Lammer, a reported £197m windfall.

Following a failed stockmarket fundraising in 2007 as the financial crisis started, they have agreed to sell a large part of their combined stake of 60 per cent, but will keep a minority interest in the Abingdon firm.

The pair, who met at Oxford University as engineering research students, are thought to have spent about £50,000 to set up the company in 1985 in Dr Hruska's house in Kidlington. They later moved to Haddenham, and then to Abingdon in April 1990 with six staff.

Sophos has since grown to 1,600 staff in offices across the world, including about 600 in Abingdon. It has about ten per cent of the market, with more than 100 million users in over 150 countries.

The firm provides businesses like Tesco, Panasonic, Heinz and Toys R Us with software to protect against viruses and spyware as well as firewalls and data leak prevention.

Unlike its US rivals Symantec and McAfee, Sophos does not sell to personal computer users, only to businesses.

Revenues for the 12 months to March 31 were above £171m, compared with £140m the previous year.

Mr Hruska said: "This transaction is good for Sophos, it is good for the employee shareholders, it is good for the investors and yes, it is good for the founders.

"The computer security field continues to expand with the quantity of threats increasing rapidly."

He said prospects for international growth were "excellent" and would continue to be pursued "vigorously".

"Peter and I remain as directors and substantial shareholders in the company and will be doing everything in our power to contribute to its success in the future," he added.

Chief executive Steve Munford said the deal was "a testament to the efforts of all those who helped bring Sophos to where it is today”.

According to the Financial Times, Apax is buying about 70 per cent of the company's equity, using debt to finance a third of the deal.

Sophos has won several Queen's Awards for Enterprise and the founders were joint winners of the Oxfordshire Business Person of the Year in 2002.