Former British prime minister Tony Blair will today face questioning by a House of Commons committee into the role he played in offering letters to more than 200 republicans telling them that they were not wanted for prosecution.

The existence of the letters became public after the prosecution of Donegal man John Downey on charges that he murdered four British soldiers in the Hyde Park bombing in 1982 was stopped because he had wrongly been told that he was not wanted.

The House of Commons Northern Ireland Affairs Committee has tried for nearly nine months to get Mr Blair to appear before it. Eventually it had to resort to using a rarely issued summons.

East Belfast Alliance MP Naomi Long said the inquiry had been given contradictory information about the letters, with some quarters saying they were merely a statement of fact, while others arguing that they were a critical confidence-building measure for republicans.

"They can't have been both. We would like to know the importance that he put them on them and why, particularly since letters between him and the attorney general of the time show that he was told that they were not a good idea," Ms Long told The Irish Times.

Mr Blair is expected to face strong criticism from Democratic Unionist MPs Ian Paisley and David Simpson and from Independent North Down MP Lady Sylvia Hermon, who have all been sharply critical of the letters.

Labour MP David Anderson said he had been "disappointed" by Mr Blair's repeated failures to accept invitations to appear before the committee, including a seeming unwillingness to give evidence by video-link.

"I am disappointed by that, despite being a member of the same party, but parliament had to make sure that he did come. I want to hear why he thought these letters were justified," Mr Anderson told The Irish Times.