The news comes on top of continued attempts by the Revenue to persuade as many as 10 million people to pay extra NI contributions to cover a mistake that jeopardises their right to a full basic state pension, the The Times reported.
On top of this over one million self assessment forms are still outstanding since the regime began seven years ago.
The NI figures emerged when paymaster general Dawn Primarolo responded to a written question from Liberal Democrat MP, Norman Lamb.
She explained that NI debts may be written off once it becomes ‘neither practical nor cost-effective to pursue the debt’.
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Five million taxpayers are ow using digital personal tax accounts (PTA) as part of the making tax digital strategy, HMRC said