The DUP has insisted the On The Runs letters were in effect an amnesty for IRA suspects like John Downey.
Democratic Unionist Nigel Dodds described the scheme as a "grubby deal" between the Blair government and Sinn Fein - and said it was one of the worst examples of "political chicanery".
Speaking after the publication of Lady Hallett's review, he told Westminster: "This was a shameful episode in the history of the so-called peace process."
John Downey, 62, from Donegal, was accused of murdering four soldiers in the IRA's Hyde Park bombing in 1982.
His trial was halted at the Old Bailey in February after Mr Justice Sweeney found he had been wrongly sent one of the Government's letters of assurance in 2007 when in fact the Metropolitan Police were looking for him.
"There was no parliamentary or public approval and at times Parliament was deliberately misled," said Mr Dodds.
"Lady Justice Hallett concludes no general amnesty. Certainly, as far as our party is concerned and the parties in Northern Ireland, there's no question of any amnesty, immunity or exemption for prosecution ever being acceptable whether through legislation or through the back door.
"However for John Downey and it appears now two others a possibility. The fact was that it was an amnesty for them."
First Minister Peter Robinson said the report exposed serious failings within the Northern Ireland Office.
He added: "This scheme was wrong in principle and shambolic in practice."
Referring to Northern Ireland Secretary Theresa Villiers' pledge to seek further legal advice on the status of the letters, Mr Robinson said it was crucial no one could use the letters in the future to avoid questioning or prosecution.
He also noted Lady Justice Hallett's assessment that the letters were not amnesties or get out of jail free cards.
"And in the legal definition of those terms she is of course right," he said.
But he further claimed her assessment was not correct in respect of the Downey case and may not be the case for the other two individuals that the report found had been sent letters in error.
He also expressed concern about those who had received a generic letter of assurance that did not contain a caveat that they could still be prosecuted if further evidence emerged.
"Of course it was a get out of jail free card for Downey," he said.
"It has the potential of being a get out of jail card for the two that Lady Justice Hallett refers to as in error, it has the potential of being a get out of jail free card for the others who were not given the caveat that was entered in some of the personal letters that were sent out. And indeed it has the potential of being a get of out jail free card for anyone who has received a letter because no one knows whether it would amount to an abuse of process application being accepted by a future judge.
"Therefore it becomes absolutely essential, and the heart of this issue is - what happens next?
"What will the Secretary of State be advised to do by her legal advisers and will she withdraw any aspect of the scheme that allows someone to rely on those letters by way of avoiding, not just prosecution, but avoiding questioning that might lead to prosecution."
In February, the DUP leader had called for the names of the OTRs to be made public.
Today he said he had to accept legal opinion that to publish the names might potentially jeopardise future criminal proceedings.
Ms Villiers said that despite the conclusions of the review it will be virtually impossible to lift the stay on Mr Downey's case.
"I'm afraid the legal advice is it's virtually impossible for there to be circumstances to arise where that stay could be lifted so I'm afraid that decision is irreversible," she said.