HMRC believes half of tax advisers’ fee income is earned from tax avoidance.

The astonishing claim, which advisers said showed the taxman was ‘living in cloud cuckoo land’, emerged from details of its ‘tax gap’ calculations, released on Budget day last week.

In a paper written in April 2005 and released following a Freedom of Information request, HMRC split tax advisory work into three types: tax computations, tax planning and avoidance.

Using an unspecified ‘detailed methodology’, HMRC concluded: ‘The result is that some 50% of total fees are earned from avoidance.’

On the assumption that for every £1 paid in avoidance advisory fees to an accountant, £10 is avoided in tax, total tax avoidance costs around £10bn.

Grant Thornton’s Mike Warburton said: ‘We certainly do tax planning work, but we are not spending half our time doing schemes. They are living in cloud cuckoo land if that’s their idea.’

Deloitte partner John Cullinane said: ‘None of the categories seem to pick up most of our advisory work on commercial transactions.’

A spokesman for HMRC said: 'The figures for the tax gap in the 2005 study are highly speculative which is why we were reluctant to release them. The use of artificial avoidance schemes has become less attractive in recent years because of action by the Government, which would be consistent with a decline in the proportion of fees arising from 'avoidance work'.