The comfort letters issued to the so-called republican on-the-runs (OTR) must be declared "null and void" so that pledges of non-prosecution should have no legal standing, First Minister Peter Robinson has told a Westminster select committee.

Mr Robinson also expressed "suspicion" that senior republicans such as Gerry Adams and Deputy First Minister Martin McGuinness received official British guarantees that they would never face prosecution for alleged involvement in actions such as the killings of Jean McConville and Frank Hegarty.

Mr Robinson said the OTR scheme was a "dark chapter in the chronicles of [British] government conduct" when he gave today evidence on the issue to the House of Commons Northern Ireland Affairs committee.

The DUP leader said the OTR scheme which benefited 228 republicans was “inequitable” and “sectarian” in that it was a concession to republicans alone.

The committee over two days of hearings at Stormont heard evidence from various politicians and from policing, legal and victims' representatives including from the North's Director of Public Prosecutions, Barra McGrory QC, who addressed the committee today.

Controversy was triggered in February when the trial of Co Donegal man John Downey for the 1982 IRA Hyde Park bombing in which four British soldiers were killed was halted, because he had mistakenly received a letter saying he did not face prosecution.

As a result the House of Commons committee is investigating the OTR scheme, while British prime minister David Cameron ordered a separate private inquiry to be carried out by Lady Justice Heather Hallett which is due to report at the end of this month.

Mr McGrory said he believed comfort letters issued to the on-the-runs would not be an impediment to future prosecutions if new evidence were to emerge against the recipients of the letters.

He said the recipients “ought not to be sleeping easily in their beds” if new evidence came to light that would make them liable to prosecution.

Mr Robinson however in his evidence noted that Mr McGrory was speaking as a prosecutor. “If the DPP had been speaking as a defence counsel he might have said something entirely different,” he said

There could be future defence claims of an abuse of legal process and therefore “the letters need to be made null and void”, insisted Mr Robinson.

Asked by Lady Hermon MP whether he had confidence in Deputy First Minister Martin McGuinness, who would have known about the scheme since its initiation in 1999, he responded, "I always expect republicans to behave like republicans. I don't expect them to do things in the interest of the wider community."

Mr Robinson was also asked by Labour MP Kate Hoey whether he was aware of any commitments given to senior members of Sinn Féin such as Gerry Adams that they would never face prosecution.

In his earlier evidence, Mr McGrory, who prior to being appointed DPP acted for Sinn Féin and was aware of the OTR scheme, said republicans such as Gerry Adams, Martin McGuinness and Gerry Kelly had not received these letters of comfort.

Mr Robinson in his response to Ms Hoey said he was suspicious that other non-prosecution commitments were given to leading Sinn Féin figures. He said this was "fuelled" by what followed events such as the 1986 IRA murder of alleged informer Frank Hegarty from Derry and the 1972 murder of Jean McConville, about which Mr Adams was recently questioned.

On Mr Hegarty’s murder Mr Robinson said, “There was certainly a considerable amount of circumstantial evidence about it in [British Central Television’s] The Cook Report…and how he was inveigled into coming back to Northern Ireland by Martin McGuinness.

"Mrs Hegarty herself indicated that he [Mr McGuinness] got down on his knees and pleaded with her to bring her son back [from Britain]. Of course when he came back he was taken and left in a cold laneway in the Irish Republic, dead. That may not be enough to prosecute, but it certainly is enough to ask questions."

Mr McGuinness has denied these allegations while Mr Adams has also denied any involvement in Ms McConville’s murder.

During today's hearing, DUP MP Ian Paisley described Mr McGrory as a "poacher turned gamekeeper" because he was now DPP, while previously he was a solicitor acting for clients such as Sinn Féin.

Mr McGrory said it was not a term he would like to use while adding, “Sometimes poachers who turn gamekeepers are more effective gamekeepers.”