Sophos, the Oxford-based IT security company, is planning to list on the London Stock Exchange in a move expected to raise about £100m ($209m) and value the company at between £300m and £500m.

If successful, it would be the second technology float in London since turmoil hit the markets this summer and follows the float of Telecity, the data centre company, last month. It will also be the second attempt at a technology listing by TA Associates, the venture capital group, after its failure to bring SmartStream, the banking software company, to market last month. TA also has about a 20 per cent stake in Sophos, bought for £41m ($60m) in 2002.