Gordon Brown expects to raise more than £4bn in 2006-07 from the clampdown on tax loopholes and fraud.

More anti-avoidance measures are expected in next week's Budget, the chancellor's ninth.

"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.

However, the closure of one loophole tends to encourage businesses that are intent on reducing their tax bills to seek another.

Last December the government announced it would retrospectively close down income tax avoidance that it deemed abusive. But the time lags between identifying a scheme and shutting it down meant the Exchequer was losing money continually.

The government has tried to improve its information on tax avoidance. It recently set up a new anti-avoidance unit within the Revenue and Customs department, which is designed to collect intelligence on schemes and decide when to intervene.

The Treasury has also been able to scrutinise tax avoidance schemes far more effectively since the introduction last autumn of a new tax regime, under which businesses were forced to disclose their tax planning ideas to the Revenue earlier than before.

Its continued success on this front is not assured. The government's attempt to raise revenues by tackling avoidance will inevitably be countered by the tax planning industry.

Alexander Klemm of the Institute for Fiscal Studies said: "Globalisation and technological change has made it easier to avoid paying taxes so you have to introduce more anti- avoidance measures just to stand still."

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Published: Thursday, March 10, 2005