This comes in addition to the 6m people who were told in September that they had paid incorrect amounts of tax.
However, 250,000 state pensioners have been told their tax underpayments for those two years will be written off.
The update on the PAYE saga was given to MPs by the Exchequer Secretary to the Treasury, David Gauke, on Tuesday.
The income tax underpayments by the 250,000 pensioners, for 2008-09 and 2009-10, have just come to light.
However, Mr Gauke made it clear there would be no attempt to ask them to pay.
"These pensioners have not yet been issued with a notice of underpayment but would have a strong case for their underpayment to be written off in line with the [extra statutory] concession," he said.
"HMRC will not require these pensioners to claim the concession individually, but will instead write off all the relevant underpayments," he added.