Former Labour prime minister, Tony Blair has finally bowed to demands from House of Commons MPs to appear before an inquiry investigating the decision to give 200 Republicans, including Donegal man, John Downey, letters telling them they were not wanted for prosecution.

The Commons' Northern Ireland Affairs Committee has sought to probe Blair's actions during his time in No 10 Downing Street since last March, but faced months of delays and obstruction from Mr Blair, who claimed that he has nothing new to say to them, and that his travel schedule made it difficult for him to find time to appear before them.

Last month, MPs sent a rarely-issued summons to the former Labour PM, making it clear that he could be held in contempt of parliament if he did not turn up on January 14th to give evidence to the inquiry, which is nearing the end of its investigation into the on the runs letters.

Meanwhile, prime minister David Cameron and Northern Ireland Secretary Theresa Villiers' continue to bar two ex-Northern Ireland Office officials from appearing before the committee this afternoon to give evidence about their role in giving out the letters.

One MP, Lady Sylvia Hermon said the efforts to keep the former Northern Ireland Office officials from testifying were "absolutely indefensible".

Shortly after noon, Ms Villiers wrote to MPs once refusing to allow the officials to appear: “This letter makes clear that while the Government fully respects the right of the Committee to inquire into any matter of its choosing and to seek evidence where it wishes, it is an important point of principle that it is Ministers rather than officials who are accountable to Parliament for the policies, actions and decisions of their departments.

"However, in the light of the Committee's latest request, the Secretary of State has indicated that she would be willing to appear again alongside Sir Jonathan Stephens, and she will also invite the officials that the Committee have asked to see to attend with her," said a NIO spokesperson.

One of the officials, Mark Sweeney, who now works with the Cabinet Office in Whitehall, personally signed the letter that was wrongly issued to John Downey, who was wanted for prosecution on charges that he murdered four British Army soldiers in the 1982 Hyde Park bombing.

His prosecution was stayed by an Old Bailey judge, who declared that the Donegal man could not be tried because he had relied on the letter of comfort issued in 2007 to travel through Gatwick Airport in 2013 on the way to a Greek holiday.

The top official in the Northern Ireland Office, Sir Jonathan Stephens was subjected to a barrage of criticism from MPs about Villiers’ decision - one that has been personally backed by Mr Cameron - when he appeared before the NI Affairs Committee on Tuesday evening.

Mr Stephens, the NIO's permanent secretary, was not at the helm when the issues were issued and could not "fill in the gaps, or shed light" on the actions that were taken by Sweeney or by his colleague, Dr Simon Case.

Up to now, the NIO has argued that "junior officials" are not put before MPs. However, Mr Stephen's case was weakened after the NI committee chairman, Laurence Robertson told him that Mr Sweeney had recently appeared before another Commons committee.

Clearly angry about the attitude being adopted by Mr Cameron and Ms Villiers, Mr Robertson said the July 2007 letter by Mr Sweeney had been “sufficient to put a stay on” on a murder prosecution. “How junior is that?” he demanded.

Illustrating the temper of MPs about the refusal, Democratic Unionist MP, Ian Paisley said clashes between the parliament and the executive dating back to the 16th Century have usually ended "with the king losing his head".

The decision by Mr Cameron and Ms Villiers "gives off an awful pong" and backs up the belief held by many that a deliberate conspiracy existed between the Labour government and Sinn Fein to ensure that IRA murderers never faced justice.

Repeatedly accepting that “catastrophic errors” were made in the Downey case, Mr Stephens told MPs that they had meant that an individual “who should have been brought to justice was not brought to justice”.

Mr Sweeney, the former Head of Rights and International Relations Division at the NIO, and Dr Case, the former Deputy Director of its Security and Legacy Group, are scheduled to appear before the NI Affairs Committee at 2.30pm on Wednesday.

The officials face the danger that they will be found to be in contempt of parliament - a rarely-implemented, but serious offence - “through no fault of their own”, Mr Robertson told Mr Stephens: “In any event, what is there to fear?” he asked.