"HMRC has a growing stack of individuals that it is confident it can prosecute," he said. "Its new investigatory powers and the reams of data that HMRC gets from private banks, letting agents, the Land Registry and accountants means it is not short of leads. That means it is less enthusiastic – or even willing – to offer plea bargain agreements."
"Plea bargain agreements have become increasingly popular with taxpayers in recent years. In the past, only those with very large tax exposures used these agreements but now those who owe much smaller sums are applying for them," he said.
HMRC has received information on 5.7 million offshore bank accounts held by UK taxpayers since the introduction of the Common Reporting Standard global data-sharing initiative in 2016-17.
Porter said that regardless of the success of information exchange initiatives, HMRC should continue to market the contractual disclosure facility and similar schemes. The tax authority collected £95.8 million in tax through agreements under the scheme last year, up from £91.1m in 2017-18, he said.
"These immunity deals are a powerful incentive to those who would otherwise have little reason to come forward," he said. "They also generate very substantial amounts of revenue without all the cost of investigations and prosecutions – over £200,000 per case."