Plumbers and gas fitters are being targeted in a campaign by the UK tax authority aimed at collecting unpaid taxes.
Heating engineers and other similar tradespeople are also being told to declare their earnings and pay tax by HM Revenue and Customs (HMRC).
If they make a full confession of past unpaid tax then their penalty should be just 10% of the unpaid money.
The campaign follows similar HMRC tactics used with doctors and dentists.
Those who want to take advantage of the offer have to contact the authority by 31 May. They must then pay the past tax, plus interest and penalties, by 31 August.
Those who come forward before the deadline would only have their records from the last five years studied by HMRC rather than the current norm of 20 years, unless criminal wrongdoing is suspected.
Anyone who does not come forward, but who is subsequently investigated and found to have been avoiding tax, may be fined up to 100% of their unpaid tax.
Cash in hand
An HMRC spokesman said that some of the 200,000 registered plumbers and gas fitters in the UK were moonlighting, being paid cash in hand and failing to pay tax.
These were the people being targeted by cross-referencing tax records with information offered from other agencies, he added.
"Our aim is to make it as easy as possible for plumbers to come forward, make a full disclosure and benefit from a reduced penalty," said Mike Wells, of HMRC.
"We will be using various intelligence sources to target plumbers who have not declared their full income. I strongly urge any in this group who think they may owe tax on their income to get in touch with HMRC and get their tax affairs in order simply and on the best available terms."
Gary Ashford, of the Chartered Institute of Taxation, said that HMRC would be likely to offer the same deal to any professionals who came clean about their tax affairs.
"Despite the focus on plumbers, HMRC has effectively announced a general disclosure facility open to anyone with tax irregularities," he said.