SINN Fein is considering taking legal action following yesterday's decision by Miss Betty Boothroyd, the Speaker of the House of Commons, to ban their two MPs from using the facilities at Westminster unless they take their seats.
This would involve swearing the oath of allegiance to Queen Elizabeth.
Mr Martin McGuinness, MP for Mid Ulster, said the House of Commons' rule book states that he is entitled to all the privileges.
"I think it raises very serious questions about the attitude of the British parliament in relation to treating the people that I represent as second class citizens. There may be legal avenues we would need to explore," he said.
To cheers from MPs, Miss Boothroyd told the Commons that she had decided to extend the restrictions barring the two men "in the interests of the House" from using the facilities. The new rules will apply from Tuesday.
Acknowledging that the British parliament had "traditionally accommodated great extremes of opinion", Miss Boothroyd added: "Equally, I feel certain that those who choose not to take their seats should not have access to the many benefits and facilities now available in this House without also taking up their responsibility as members."
The Sinn Fein president, Mr Gerry Adams, MP for West Belfast, said he was not surprised by the decision but that they would both go to Westminster next week.
"It is not the end of the world. Part of the silliness about this is that Sinn Fein is in no way dependent on the facilities, though we have the right to them," he said. He and Mr McGuinness will be "there because there is a gap in our schedules and to pick up our passes".
However, the former Northern Ireland minister, Mr Michael Ancram, welcomed the decision, arguing that it would have been "incongruous and incredible" if Sinn Fein had not been barred. He pointed out that other nationalist MPs swore the oath and then argued their cases in parliament.
"These men, McGuinness and Adams, are inextricably linked, because they are the political wing of the Provisional IRA, with men who seek to use violence for political ends and to undermine the institutions of the state, including the parliament they're now trying to use the facilities of," he said.
The Rev Ian Paisley called for the ban on Sinn Fein to be widened. "If Sinn Fein/IRA candidates and elected representatives are not good enough to stroll the corridors of power in Westminster, then they are certainly not good enough to stroll the corridors of power in Northern Ireland," he said.