HMRC has written to 28,000 doctors suggesting they make use of a tax 'amnesty' before the whole medical profession's accounts are scrutinised later this year. But it rejected calls to extend the amnesty deadline, despite fears GPs are too busy with end-of-year paperwork to respond.

As part of a crackdown on tax avoidance, HMRC is offering the chance for doctors who may have underpaid tax in the past to rectify the situation.

If they do not notify HMRC by 30 March 2010, those found to owe tax on income going back up to 20 years could face a penalty of 100 per cent of the tax owed, on top of the debt and any interest on it.

Accountants say HMRC must already have information on some doctors it believes have made errors or omitted data when filing tax returns.

Valerie Martin-Long, a director at PKF accountants, said it was 'pretty obvious' HMRC is 'writing to those doctors and consultants it suspects have not declared all their income - and it probably has the evidence to prove it'.

A spokeswoman for HMRC said it did hold data that 'shows us where there is a chance that something may have been omitted'. But writing to some doctors 'does not mean we are implying any wrongdoing', she added.

HMRC was writing to 'let people we think are most likely to be interested know about the upcoming deadline', she said.

Accountants RSM Tenon fear 'a significant number' of GPs have been contacted by HMRC but may be too busy to reply.

Gary Ashford, a director at the firm, said the amnesty's deadline fell just as GPs were completing the year's QOF and enhanced services paperwork.

He said: 'Early signs are that not many medical professionals are taking up the disclosure facility offered by HMRC.'

A spokeswoman for HMRC said notification was 'straightforward', requiring only the GP's name, address and GMC number, and the deadline would not be extended.

Possible unpaid tax must be declared by 31 March, but doctors then have until 30 June to make detailed disclosures.