The shares sold are likely to belong to existing investors in Sophos, which analysts at IDC describe as the largest unlisted company in the $13.2bn secure content and threat-management market.

Sophos reported billings of $167m in the year to March 31 with a growth rate of 23% which it said represented an increase in market share in an industry that enjoyed an 11.8% growth rate. In the six months to September 30, growth increased to 30% with billings at $93.7m.

Though a second-tier player in a consolidating sector, Sophos said its products are used by 65,000 customers in over 125 countries to protect 100 million end-users.

It said its endpoint security and network access control solutions simplify security and an integrated defence against malware, spyware, intrusions, unwanted applications, and policy abuse. It also offers email security and web security products that filter traffic for security threats, spam and policy infringements.

It said its subscription-based business model provides enhanced visibility and predictability. When an order is booked, the customer pays the full amount of the subscription price and revenue is then recognized rateably over the full length of a contract that can last between one and five years.

Sophos chief executive Steve Munford said a listing will enable the company to maximize its profile and take advantage of the opportunities within the market.

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A listing will also increase its vulnerability to a takeover from one of the giants of the industry and this prospect will increase the attractiveness of its shares.