HMRC has warned plumbers have until 31 August to tell HMRC about any tax they owe, and to make arrangements to pay any interest and penalties.
They were offered the Plumbers Tax Safe Plan (PTSP) in March this year, when they could put their tax affairs right and face a penalty of 10% instead of 20%. They then had until 31 August to arrange for payment.
HMRC has since begun using information held on people working in the plumbing industry to clamp down on those who have chosen not to come forward.
HMRC has left the PTSP disclosure route open for those in the plumbing industry with unpaid tax who still wish to come forward, though they may not receive the same terms as under the plumbers plan.
Since June, HMRC has been using information gathered from many different sources to open investigations into hundreds of people who have failed to come forward. In addition to penalties, defaulters can face criminal prosecution.
Mike Wells, HMRC's director of risk and intelligence, said: "Plumbers who have notified us of their intention to disclose now have until the end of August to tell us what they owe and make arrangements to pay.
“I urge anyone with unpaid tax to use it to come forward and avoid potentially lengthy and costly investigations. The penalty they will pay will still be lower than when HMRC catches up with them.”
Disclosures can be made online at www.hmrc.gov.uk/plumberstaxsafeplan, or by calling HMRC direct on 0845 600 4507.
HMRC campaigns have so far raised more than £500 million from voluntary disclosures and a further £100 million from follow-up investigations. Campaigns have targeted offshore investments, medical professionals and those trading above the VAT turnover threshold but who have not registered for VAT.