Donald Dewar, the Labour Chief Whip, read the riot act individually to three Labour MPs - Ken Livingstone, Jeremy Corbyn, and Alan Simpson - in his room off the members' lobby.
The Chief Whip said it was a matter of some sensitivity. It had been drawn to his attention the MPs had put the House at some considerable and unacceptable risk.
A Labour source said: "He had been informed by the security services that people with Mitchell McLaughlin were directly involved with the IRA or connected with the IRA. He said this House had been the target in the past and could well be in the present and the future."
Mr Dewar told the MPs that having the Sinn Fein members in the Commons had placed their Labour colleagues and the whole House at risk. One of the three said: "He told me I had allowed them to roam freely around Parliament and the security services had closely monitored their movements and wanted him to know of the risk. He appeared to have a report.
"My response was just to point out that Mitchell McLaughlin had initiated it. He had already met a large number of members and that I simply had fitted into a slot where I was available that afternoon and they were already involved in other meetings."
"Plods", the cafe off the Great Hall used by the Commons police, seemed the perfect place for Sinn Fein leaders to meet MPs. It was quiet, and it was easy for MPs to come and go.
The security service, MI5 , yesterday took the highly unusual step of issuing a statement denying that they had contacted Labour about the Sinn Fein visit or had been involved in a surveillance operation at the Commons. Labour MPs had been under the impression that Mr Dewar was acting on information supplied by MI5. A security source said: "MI5, categorically denies that it had any involvement in the events alleged in the story ... We were not involved in any way."
Selected Labour MPs had been invited by Mr McLaughlin to join them to discuss their agenda for getting Sinn Fein into the Ulster peace talks. They did not realise they were being watched from a nearby table by a group of security officers from the House of Commons, dressed in casual clothes.
The importance of security was etched in the walls at the police canteen. The stonework is still scarred by an IRA bomb attack more than a decade ago.
Mr McLaughlin and his two friends had spent a busy day touring all the main MPs' office blocks, where they had meetings with individual Labour MPs in their private office suites. They had been to Millbank, One Parliament Street and the Norman Shaw building. "They were trying to let people know that a lot of work was going on to reconstruct the ceasefire," said one source.
The decommissioning of IRA weapons - the issue on which the peace talks could founder - was hardly discussed. But the Labour MPs who met the Sinn Fein group found they had all given the same message: there would have to be a ceasefire before Sinn Fein could hope to get into the talks.
One MP who met them said: "We left them under no illusions that the left in Britain were absolutely adamant that without the restoration of the ceasefire, there would no peace process and they needed to take that message back to the hard men in the IRA.
"We all told them that what the IRA were doing was eroding the traditional support Sinn Fein had in Britain. We all told them the same thing: they would be throwing away the republican case by seeking to unleash another wave of indiscriminate bombings."
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