Alan Clark, the senior Tory MP and historian, last night launched a surprise attack on the law barring the leaders of Sinn Fein taking up the places to which they were elected in May.
Mr Adams, the elected MP for West Belfast, and Mr McGuinness, the MP for Mid Ulster, are currently engaged in all-party talks on the future of Northern Ireland following the reintroduction of the IRA ceasefire.
As one of the party leaders engaged in the talks, Mr Adams is scheduled to meet Tony Blair for discussions at Number 10 before Christmas, on the same basis that David Trimble, the leader of the Ulster Unionists, had talks with the Prime Minister in Downing Street yesterday.
But Mr Adams and Mr McGuinness have been barred from the precincts of the Commons on a ruling from the Speaker, Betty Boothroyd, because they refuse to affirm their allegiance to the Crown.
While it is the law that no MP can sit, vote or speak in the Commons without having sworn or affirmed allegiance - under the terms of the Parliamentary Oaths Act 1866 - without prompting a by-election, there is no law under which such dissenting representatives are barred from the precincts or facilities of the Commons.
That was the decision taken by the Speaker when it became clear that neither man would affirm allegiance to the Crown.
But in a characteristically independent intervention in a second reading debate on the Northern Ireland (Emergency Provisions) Bill, Mr Clark challenged the law - arguing that it played into the hands of the nationalists who wanted to opt out of the United Kingdom. He said: "If we deny the right, effectively remove a constituency from the United Kingdom, we are in effect encouraging all those who argue the case that it should no longer be a part of the United Kingdom ...
"As for the oath, which would the House regard as being more honourable; to refuse to take it on grounds of conscience or commitment, or to take it as did the honourable Member for Newham North West [Tony Banks, now minister for sport] smiling at the television cameras, with his fingers crossed?"
Following the Speaker's ruling, the two MPs tried and failed to get a judicial review of her decision - but the courts have no power to interfere with the domestic affairs of Parliament, theoretically at least, the highest court of the land.
It has been suggested that they might try to get a ruling from the European Court, but there have also been reports that Miss Boothroyd has suggested a meeting.
The Commons last night gave an unopposed second reading to the emergency provisions legislation, which scraps internment powers and maintains the anti-terrorist legislation - pending the introduction of permanent UK-wide anti-terrorist laws, expected in 2000.
Adam Ingram, the security minister, said the loyalist ceasefire was holding, while Ronnie Flanagan, the Royal Ulster Constabulary Chief Constable, believed the IRA ceasefire "remains remarkably stable". But the minister warned that some terrorist groups "on both sides remain active and could destabilise the peace process".