Traders who charge VAT when they are not registered and pocket the tax should be jailed, an MP has said.
Ian Liddell-Grainger, chairman of the All-Party Parliamentary Taxation Group, described the scam as "blatant fraud".
He said the penalties for cheating on VAT are currently very low - usually no more than double the tax charged.
HM Revenue & Customs (HMRC) said it was well aware that VAT numbers could be used for illegal purposes and monitored their use closely.
'No deterrent'
Mr Liddell-Grainger, the Conservative MP for Bridgwater, told BBC Radio 4's Money Box: "Everyone knows it is going on but it is hard to detect. Now VAT has gone up to 20% it will become much more high profile."
And he called on the government to increase the penalties.
End Quote Rick Crees Association of Plumbing and Heating ContractorsSourcing your tradesman through a recognised trade association is a good way to dramatically reduce the risk of hiring someone dishonest”
"Double the tax is no deterrent. If you've got a tax bill of £30, that is £60. In the worst cases it needs to be a custodial sentence, and certainly up to 10 times the tax," he said.
"This is blatant fraud. You are going into people's homes, lying and using your position to extort money out of people. In any other walk of life you would be jailed."
The fraud is simple. A trader either does not register for VAT or, if they are registered, they deregister for VAT. But they still add 20% onto the bill and call it VAT. Instead of passing that amount on to the tax authorities, they pocket it themselves.
Building firm boss Nigel Leck told the BBC that such practices were growing and he now checks every invoice he gets from contractors.
Mr Leck, who runs Property Presentation Services in Warwickshire, said: "Twice we have discovered people trying to charge our customers VAT when they are not registered, or they have deregistered.
"They know they are not registered and they're still charging the VAT."
Mr Leck says he now always checks all the paperwork from every company.
He uses a European Union website, which checks a VAT number and shows whom it is registered to. Although intended for businesses doing cross-border business, it will provide the answer instantly to anyone.
Alternatively, individuals can call an HMRC helpline - 0845 010 9000 - to check registration. There is also an HMRC hotline to report fraud on 0800 595 000.
'Damaging industry'Mr Liddell-Grainger recommends using these services rather than challenging someone who is in your home and expecting payment for a bill showing VAT.
"People can check on the website to see what the situation is, and report it to HMRC. That hotline should be used," he said.
Rick Crees, a spokesman for the Association of Plumbing and Heating Contractors, said the practice was not widespread.
"Any sole trader, whatever their business, could be trying this trick. It is perfectly possible that plumbing and heating engineers are among those trying it. This kind of dishonesty is damaging to our industry," he said.
"Sourcing your tradesman through a recognised trade association is a good way to dramatically reduce the risk of hiring someone dishonest."
Any trader whose turnover exceeds £70,000 must register for VAT. Those with lower turnovers can do so, but do not have to.
"HMRC is well aware that VAT numbers can be used for illegal purposes and we monitor their use closely," a spokesman for HMRC told Money Box.
"We have a range of penalties, including criminal prosecution available to tackle any abuse."