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Following revelations in last week's Computer Weekly that taxpayers were told they had been using the wrong national insurance numbers, the Inland Revenue claimed this was a result of "routine" data cleansing rather than any problem with the systems.
But now an internal Revenue journal, Insight, has revealed that the Revenue was due to bring down parts of the main departmental PAYE systems for two weeks to "remove duplicated national insurance numbers and put right errors" in advance of the introduction of new tax credits.
"[With new tax credits being introduced] it is vital that Revenue systems have the right customer details held under the right national insurance number," the journal said.
The problems with national insurance numbers were first highlighted on BBC Radio Four's Moneybox programme, after it was contacted by IT specialist Sarah Wilkinson. She said she had been using the same national insurance number all her working life but was being told by the Revenue to use a new "correct" number.
The letter was one of hundreds of thousands sent out by the Revenue after an internal data cleansing exercise. When several records for an individual were found under different national insurance numbers, a new computer system, codenamed CR74, recommended a single number to use in future.
Wilkinson said this week, "The implication in the letter is that I am responsible, but it now seems that the problem is widespread, which is even more worrying. My concern now is that for the last 20 or so years I have been paying contributions to a national insurance number possibly shared by someone else. I am not sure that I trust the Inland Revenue to sort this out."
An Inland Revenue spokeswoman said the problems would be corrected. Anyone who is worried should contact their tax office, she added.