British prime minister David Cameron and Northern Ireland Secretary Theresa Villiers have barred two officials from giving evidence on Wednesday to a House of Commons inquiry into letters telling republicans that they were not wanted for prosecutions.

Members of the Northern Ireland Affairs Committee have expressed fury about the decision, with one MP, Lady Sylvia Hermon, saying that efforts to keep former officials of the Northern Ireland Office (NIO) from testifying were "absolutely indefensible".

One of the officials, Mark Sweeney, who now works with the cabinet office, personally signed the letter 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. Mr Downey's prosecution was stayed by an Old Bailey judge, who said 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 holiday in Greece.

The top official at the NIO, Jonathan Stevens, was criticised by MPs about Ms Villiers’s decision, one personally backed by Mr Cameron.

Mr Stevens, the NIO’s permanent secretary, was not at the helm when the letters were issued and, appearing before the committee, said he could not “fill in the gaps or shed light” on the actions that were taken by Mr Sweeney or his colleague, Dr Simon Case.

Up to now, the NIO has argued that “junior officials” are not put before MPs. However, this case was weakened when committee chairman Laurence Robertson told Mr Stevens that Mr Sweeney had recently appeared before another Commons committee.

Angry at 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 a murder prosecution. “How junior is that?” he asked.

Illustrating the temper of MPs about the refusal, Democratic Unionist MP Ian Paisley jnr said clashes between 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 backed up the belief held by many that a conspiracy existed between the Labour government and Sinn Féin to ensure IRA murderers never faced justice, he said.

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

Mr Sweeney, former head of the rights and international relations division at the NIO, and Dr Case, former deputy director of its security and legacy group, were scheduled to appear before the committee at 2.30pm today.