The taxman begins an “intrusive and thorough” crackdown on tax fraudsters today by sifting through the records of 340,000 people after a tax amnesty that has netted up to £1 billion for the public purse.

HM Revenue & Customs (HMRC) vowed to pursue those who had not taken advantage of its nine-week partial amnesty and to seek much stiffer penalties than it has in the past.

Between 55,000 and 60,000 people opted to declare unpaid tax on offshore accounts under HMRC’s “offshore disclosure facility”, which closed at midnight last night. Roughly half of those left it until the last few days to call the Revenue.

Those who have disclosed unpaid tax now have until November 26 to pay an estimated £1 billion, which includes interest and in many cases a fine of up to 10 per cent of their bill. Yet tens of thousands of people with offshore holdings now risk penalties of up to 100 per cent.

HMRC has records of 400,000 people with offshore bank accounts and estimates that approximately 100,000 of those have tax to pay and are not exempted by the nondomicile laws. As a result, about 40,000 people may be at risk of a heavy fine.

An HMRC spokesman said: “We will definitely take it into account that we have put forward a unique opportunity for people to come forward. When people have ignored the offshore disclosure facility, yes, we will factor that in in calculating a penalty. We have gone out of our way to make it easy. People who think we will go away [now] are making a mistake.”

Dave Hartnett, the HMRC director-general of business relations, told Taxation magazine that his investigation team was “raring to go”. He said that their inquiries would be “intrusive and thorough”.

Prominent tax cheaters would be singled out for early criminal prosecution, he said, starting with anyone connected to the tax world such as tax advisers, judges, or even HMRC employees themselves.

Reg Day, the national director of tax investigations at KPMG, the accountants, said: “Lots of people are wrestling with whether to put their names forward. Many have irregularities extending over a long period. Their analysis is that if they come forward, their offshore nest egg is going to be cleaned out.”

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