The UK's tax authority has sent letters to account holders at HSBC Jersey in a final warning for voluntary disclosure.

HM Revenue & Customs has asked clients of HSBC Jersey to sign a certificate declaring that they owe no UK tax on their offshore assets. This includes “the source of the funds in [their] Jersey bank account”, according to the Financial Times.

If these customers do not respond by the end of the year, HMRC said it “may start a detailed investigation” into their tax affairs.

In March this year, HSBC said it was reviewing details of Jersey-based accounts held by clients in the UK while tightening up protections against potential fraud and other crime.

In the week since the letters were sent, HMRC has already heard from over 15 per cent of customers who have explained that they have or intend to make a disclosure, or have nothing further to disclose. The authority said it had also heard from agents who have explained they are using this letter as a prompt to remind their clients to ensure their tax affairs are in order.

“We are not saying that these taxpayers have done anything wrong. If they are confident that their tax affairs are above board, we ask them to fill in a certificate telling us so. If they are unsure, we suggest they seek urgent advice from a tax professional and, if tax is due, they must make a disclosure and pay any tax due,” said an HMRC spokesperson.

“But make no mistake, we are getting much tougher on offshore evasion. There will be serious consequences for those with undeclared offshore income and gains, who do not come forward voluntarily.”

The warning calls for particular attention given HMRC's plans to make tax evasion a strict liability offence so that it is not necessary to prove intent to commit a criminal offence, meaning that failure to disclose offshore income may result in to a criminal offence.

“This action by HMRC shows that they are already gearing up to deliver the big stick in the wake of the carrots that have previously been offered to taxpayers in the form of a variety of different disclosure opportunities,” Fiona Fernie, partner and head of tax investigations at Pinsent Masons, told WealthBriefing.

“Whether the taxpayer intended to evade tax is irrelevant. Whilst HMRC has stated that it will not use the offence against those who have taken 'reasonable care' in making their tax returns, recent experience suggests they will take a hard line in determining what counts as 'reasonable care'.”

HMRC will be closing its disclosure facilities on 31 December 2015.