Individuals who have registered previously undeclared accounts as part of HM Revenue and Customs' offshore 'tax amnesty' have just one month left to submit self-assessed declarations.
Undeclared interest from offshore accounts, including those held in the Channel Islands and Isle of Man, have been earmarked for investigation.
To encourage account holders to come forward, a "tax amnesty" was announced earlier this year that allowed tax payers to register undeclared accounts.
Nigel Neville, at Birmingham accountants Mazars, said: "The amnesty was organised in two stages - a registration process followed by a period to enable tax payers to gather information and self assess their liabilities.
"Anyone taking advantage of the amnesty was offered the possibility of a restricted 10 per cent penalty, as opposed to facing the usual penalty of up to 100 per cent of any undeclared tax liability, plus interest, and even criminal prosecution.
"The registration period ended on June 22 and HMRC received only 60,000 applications, a figure far short of its expectations.
"It has been suggested that HMRC has details of at least 100,000 other taxpayers who have not registered and this figure is likely to increase as more institutions are required to divulge details of offshore accounts.
"Add to this the fact that 170 institutions have been approached by HMRC recently and the implication is that there are large numbers of potential investigations in the pipeline."
He continued: "It is not clear whether HMRC has the resources to mount a significant number of investigations immediately, so it may be some time before some taxpayers are targeted. However, this merely increases the number of years open to investigation, increasing the financial risks associated with failure to declare these accounts.
"The period for self assessment ends on November 26 and it is essential that anyone who has registered submits the relevant declarations by this date as failure to make a self assessment following registration may in itself result in an investigation.
"It may also still be possible to take advantage of the amnesty, even though the initial registration deadline has now passed, if individuals can make a full declaration by then. An unprompted disclosure will reduce the exposure to penalties and provided that sufficient information is given to the inspector, may also reduce the likelihood of a formal investigation."
Two potential disincentives to disclosure have been acknowledged by HMRC - difficulty in obtaining relevant information as the disclosure may have to cover income in the 20 years to April 5 last year, and the possibility of HMRC instituting criminal proceedings following such a disclosure.
However, HMRC has confirmed that where third party information is not available they will accept estimates, assuming that the taxpayer takes all possible steps following the self assessment to obtain the necessary information and updates their disclosure as appropriate.
Similarly, it is also possible to obtain a clearance from HMRC that criminal proceedings will not be instituted.
Mr Neville said: "For those wishing to take advantage of the amnesty, it is essential that a full disclosure be made as soon as possible."