While the Revenue said that the problem only 'affects a very small number of cases', Stephen Woolfe, financial controller at manufacturing company Insul-8, said the Revenue had admitted to knowing about the problem for 'some time'.

Woolfe had completed a client's corporation tax return using the Revenue's self assessment module, which is available over the internet, and because profits chargeable to CT were below £10,000, he was 'confident' of his client not having to pay any tax.

His client's year-end was 30 June 2003, so the software split the profits and performed two tax calculations. But while it correctly assessed the first nine months at zero, the last three months was calculated at 30% tax payable. Woolfe said he was eventually told by a Revenue helpdesk worker that its software was faulty and unable to cope with split financial years.

'I was bemused to find that the software correctly assessed the first nine months at nil payable, but the latter three months apportionment was calculated at 30% tax payable,' Woolfe said. 'The Revenue's technical help desk informed me they knew nothing about corporation tax and suggested I spoke with my client's tax office.

'The tax office professed to know nothing about the software and therefore could not help me - although they too admitted to "only a smattering of corporation tax knowledge".'

A Revenue spokesman said the problem is a 'known fault' and that it should be corrected by the time Accountancy Age was published.

'This should have been explained by the helpdesk but, in this case, clearly it wasn't. On this occasion it seems the customer did not get the service they should have done and we are sorry for that,' a spokesman said.