The British Government’s decision to tell Republican on-the-runs that they were not wanted for prosecution was not “a get out of jail card”, a top British judge declared today.

In a keenly-anticipated report, Lady Justice Hallett found that the scheme used was "unprecedented and flawed", but that it was not unlawful or improper.

The on-the-runs emerged into public view earlier this year after an Old Bailey judge ruled that John Downey could not be prosecuted for the 1982 Hyde Park bombing because he had been given one of the letters.

However, today’s report has found that Downey was not the only individual to have received letters of comfort — a fact that will be seized upon by Unionists.

In one, a man was given a letter even though he was wanted for questioning by the PSNI about a crime allegedly committed in 2003 — five years after the Good Friday Agreement.

In another case, a letter was issued by the Northern Ireland Office which gave a man's birthday incorrectly, said Lady Hallett, who was asked to review the scheme after the Downey controversy erupted.

Despite the criticisms in today's report, former Labour prime minister Tony Blair and other senior Labour figures, who have argued that the scheme was a necessary "messy compromise", will see it as a vindication of their actions.

Unionist politicians charged after the Downey row broke that the OTRs scheme was an abuse of the legal process, designed to prevent proper prosecutions from going ahead.

However, Lady Hallett found that the OTR letters were not “an amnesty”, while prosecutors were able throughout to make independent decisions on whether to take cases. Representations from Labour ministers to legal officers during the life of the scheme which began in 2010 “may have come close to the line of what was acceptable” they “did not cross it”, she found.

Significantly, she found that the Downey ruling was “made on its own facts and would not necessarily prevent the prosecution of others who have received letters of assurance”.

Later, she went on: “I repeat: it was not an amnesty. There was no intention to drop prosecutions or alleged terrorists who had yet to stand trial where the test for prosecution were met.”

Significantly, too, Lady Justice Hallett disagreed with views expressed during the House of Commons’ Northern Affairs Committee’s own inquiry into the issue.

There, it was argued by some that evidence that was known to exist by police officers before an OTR letter was sent but was then regarded as insufficient to justify arrest could not be be used subsequently.

The ruling, she said, in Downey’s case “was made very much on its own facts”. The judgment to be made in any future case where letters were issued “will depend on the individual circumstances”.

In all, 228 names were put forward for OTR letters: mostly by Sinn Féin, though four were put forward by the Irish Government, while 14 names came from the NI Prison Service.

One hundred and fifty six people were sent letters, though 31 more were told “that they were not wanted in some other way”, Lady Justice Hallett said in her report.

Twenty-three people, including the Dublin-based senior Sinn Féin figure, Rita O’Hare, were told that they were wanted — effectively warning them to stay out of Britain, or Northern Ireland.

Downey should not have got a letter, the judge found today, because Police Service of Northern Ireland Detective Chief Superintendent Norman Baxter had known that he was still wanted.

She said that she did “not understand” Baxter’s failure to declare that knowledge to his superior, Assistant Chief Constable Peter Sheridan, who now heads Co-Operation Ireland.

In reply, Mr Baxter, now retired, says that he was not aware that letters were being sent to OTRs by the NIO, adding that it was not the PSNI’s job to check the UK police national computer.

It “appears highly unlikely” that Mr Sheridan was “made aware of the fact” that Mr Downey was wanted by the Metropolitan Police in connection with Hyde Park, the report goes on. The PSNI’s letter to the Public Prosecution Service about Mr Downey “failed to disclose” this fact: “I have been given no satisfactory explanation for this failure by the PSNI. “The error was compounded by the fact that the PSNI realised its mistake in 2008 and did nothing to correct it, or at least check the nature of the assurance that had been provided to Mr Downey.”

The Northern Ireland Office had “good reason” to believe on the back of the information coming to it from the PSNI that Downey was not wanted before it issued the letter to Downey in July 2007.

Thirteen people were given Royal pardons, but all of them had been escaped prisoners so their situation was similar to those who were released after the Good Friday Agreement after serving more than two years.

“We have identified no cases where the Royal Prerogative of Mercy was used as a pre-conviction pardon for an OTR,” said Lady Justice Hallett, in her 300-page report.

The existence of Northern Ireland’s political institutions was threatened after the Downey controversy erupted, following NI First Minister Peter Robinson’s fury that he had been kept in the dark.

However, Lady Justice Hallett found that the existence of the OTRs scheme was “not kept secret, or highly confidential”, while dozens of NI officials and politicians “must have known that some kind of scheme was in operation”.

The scheme was not given “much publicity” while victims’ groups were never told, but “there were sufficient references to the overall scheme....to put an astute observer on the alert”, she said.

Nevertheless, she said “there was little appetite to publicise details of the scheme unless pressed”, but she had found “no evidence” that British ministers had actively sought “to obscure the scheme from the public”. The letters given to OTRs were poorly drafted, while the PSNI was not shown a text of the document that was sent out by the NIO until December 2011.

She expressed "surprise" that First Minister Peter Robinson or NI Justice Minister, David Ford were not told of the existence of the scheme after justice and policing powers were devolved to Stormont in 2010.

But she had “detected no sinister motive” in this, she said: “The hope seems to have been that the scheme could have been brought quietly to a close”.

The OTRS scheme was “not designed, it evolved”, the judge found: “As a result, it lacked proper lines of responsibility, accountability and safeguards. “When errors came to light,opportunities were missed to rectify them and to review the scheme as a whole. There was no policy on what to do if an error was identified.”

In some cases, people were given assurances that they were “free to return” without any warning that they could be prosecuted if new information arose. In others, people were initially given such a warning, but they were later given assurances by way of another letter which failed to make any reference to changing circumstances.

The number of letters sent out increased dramatically in 2007 and 2008 after the PSNI set up a special operation to handle the cases, partly on the back of pressure from politicians “who were pressing for a speedier response”.

Concerns were voiced by some PSNI officers working on the team that the checks being made about individuals were not thorough enough “but they were not properly addressed by senior officers”.

The judge said she had found nothing that should be allowed to undermine NI’s peace process: “One catastrophic mistake has been made and it cannot be undone. “The families of those killed in the Hyde Park bombing have no choice but to come to terms with that fact, as devastating as I know it has been for them.

“Other mistakes have been made made and need correcting. But it can be done in a measured and proportionate way. No one should use my findings to make political capital,” she said.