Gordon Brown expects to raise more than £4bn in 2006-07 from the clampdown on tax loopholes and fraud after pledging "vigilance against tax avoidance" in Budget 2002. More anti-avoidance measures are expected in next week's Budget, the chancellor's ninth.

But some tax advisers are sceptical about whether the Treasury will raise what is expected. "I don't think there is any doubt that these numbers are an over- estimate," said Bill Dodwell, a partner at Deloitte, the professional services firm.