Northern Ireland Secretary Theresa Villiers said the UK Government was not prepared to stand over the factual accuracy of the documents any more, after numerous errors were flagged up in a judge-led review of the so-called on-the-run (OTR) scheme.
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But she told a Westminster committee that there were no plans to introduce legislation to change the status of the letters, insisting the most appropriate way of making clear they were no longer of value was with a public statement.
Ms Villiers outlined her intention to the Northern Ireland Affairs Committee but said she intended to make a more detailed statement to Parliament within days. She said recipients should not rely on the letters.
"If they drew some comfort from those letters in the past they should no longer draw comfort from them in the future," she said.
The issue was thrust into public prominence in February when the case against a man accused of murdering four soldiers in the Hyde Park bombing by the IRA collapsed because it emerged he had been sent one of the letters in error, when in fact police were seeking him.
The Old Bailey trial judge found that John Downey's arrest in the UK last year, when he had been told he was free to return, represented an abuse of process.
Downey, 62, from Co Donegal, denied involvement in the 1982 attack. In July, the judge-led review of the scheme, which was agreed between Sinn Fein and the last Labour administration, found that it was systematically flawed in operation but not unlawful in principle.
Lady Justice Hallett said a "catastrophic" error had been made in the Downey case but she insisted the letters of assurance did not amount to amnesties.
Under the scheme, which started running in 2000, almost 190 republicans who had left the UK received assurances they were no longer being sought