The Revenue has written to more than one million people asking if they want their tax reviewed because they may have under- or over-paid tax for 1997-98 and will not previously have received any official notification of this.
The letter follows problems involving the Revenue's Nirs2 national insurance system and links to the Computerisation of PAYE (Cop) computer system.
With a backlog of administrative work building up, the Revenue closed automatically on the Cop system 1.04 million taxpayer records which had previously been under investigation because of unresolved difficulties.
The Revenue has admitted that it cleared the records from the ICL mainframe-based PAYE computer system last year without being certain that in every case the correct amount of tax had been paid.
Now an Inland Revenue memo, published this week as an appendix to a report by the Public Accounts Committee, says officials have checked some of the 1.04 million tax records that were cleared.
The Revenue believes that in 15% of cases, the wrong amount of tax was paid.
The department estimates that 148,000 individuals have overpaid a total of £22m in tax, while 12,000 people have underpaid their tax bill by a total of £4m. But unless the taxpayers come forward to claim it, the Revenue said it will not know who they are.
If taxpayers do not seek clarification the Government stands to gain up to £18m because of unclaimed, overpaid tax.