'They're using the closing of loopholes as a clever way of packaging what is really a 10 per cent rise in corporation tax,' said Bill Dodwell, a tax partner at the firm, in The Observer.

Analysing the increase in the corporation tax take, and adjusting for improved profitability and other changes, leaves an increase of £4bn, or 10 per cent, Deloitte said.

The suggestion that the crackdown is simply a tax increase, a view echoed by the CBI, will be regarded with great skepticism by those in HM Revenue & Customs.

Ministers have in recent years moved to crack down on the most flagrant abuses of the tax system, including the use of complicated financial instruments to avoid tax and national insurance on large city bonuses.

The Big Four accountancy firms have become increasingly vocal in their condemnation of the avoidance crackdown in recent weeks, however, which has been severely restricting their revenues on tax work.

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