The Department of Finance will pencil in a €200 million gain from special Revenue Commissioners investigations in tomorrow's 2005 Budget, it has emerged.

The provision will come just weeks after the Revenue Commissioners announced that it would set up an inquiry into single-premium insurance policies next year.

It has become traditional over the past few years for the Department of Finance to include a provision for unexpected tax windfalls drawn from Revenue investigations.

Last year, the estimated figure for 2004 was set at around €150 million but the actual yield from special investigations has so far topped €700 million.

This has contributed substantially to the projected €2 billion tax overshoot with which the Minister for Finance, Mr Cowen, was greeted when he took up office last month.

A Department of Finance spokesman said yesterday that the provision being made for 2005 did not relate to any particular investigation.

"As in other years, there is an amount provided for in a tax forecast for possible proceeds from special revenue investigations. The amount provided for in 2005 is around €200 million. This sum is not related to nor is it an estimate of any likely yields from any specific investigation," he said.

The new Revenue inquiry, which will focus on possible tax evasion through the use of insurance policies in the past two decades, will be the latest in a growing line of extremely successful special investigations.

It is almost impossible to estimate how much the inquiry could bring in but some suggest that it could raise more than €1.5 billion.

More than €30 million was invested in single-premium insurance products between 1988 and 2001.

The chairman of the Revenue Commissioners, Mr Frank Daly, has said there is no evidence to suggest that this will "not be a significant investigation".

Other Revenue investigations have yielded more than €1.6 billion in unexpected funds for the Exchequer.

Close to half of this total was collected this year as part of the Revenue's investigations into offshore assets. This inquiry had yielded some €705 million by the middle of this month, although the Department of Finance has said that this "high level of once-off monies will not be factored into the base for next year".

Almost €800 million has been collected from investigations into bogus non-resident accounts, with a further €53 million yielded from the National Irish Bank/ CMI inquiry. Ansbacher inquiries have brought in €44 million.

The investigations have seen some 30,000 people make tax settlements with the Revenue.