The Conservative leader, Iain Duncan Smith has urged the Government to exclude Sinn Féin from the power-sharing Northern Ireland Executive.
Following the arrest of Sinn Féin official Denis Donaldson for allegedly having possession of official documents that could have been of use to terrorists, Mr Duncan Smith wrote to Prime Minister, Tony Blair urging him to act.
In his letter, Mr Duncan Smith said Mr Donaldson's arrest came on top of "clear breaches" of the IRA ceasefire including the break-in at the Castlereagh police station, IRA activities in Colombia, and the orchestration of street violence in Belfast.
Mr Duncan Smith noted that in his speech to the House of Commons on July 24, Northern Ireland Secretary, Dr John Reid said he would not hesitate to use his powers in the event of further ceasefire breaches.
Mr Duncan Smith wrote: "It has subsequently emerged that the police investigation - involving some 200 officers - has been on-going for some 13 months after it was discovered that a Northern Ireland Office employee was copying official documents to pass to outsiders.
"If this is so then you must have known about it when I came to see you in April and also when John Reid made his statement to Parliament in July. Why, then, did that not stiffen your attitude against Sinn Féin at the time, instead of which you declined to take any action, both before the Hillsborough talks and subsequently, against Sinn Féin breaches that were already public? "Nobody wishes to see the political process in Northern Ireland collapse.
"Yet it can only be sustained if all parties demonstrate a commitment to what the Belfast Agreement calls 'exclusively democratic and peaceful means.' The latest disclosures must now seriously call into question Sinn Féin's fitness for office.
"I am therefore urging you to act now on the commitments given by yourself and John Reid in order to preserve the integrity of democratic politics in Northern Ireland." A Tory spokesman said that Dr Reid had the power to put a motion before the Assembly for the exclusion of any party found to be in breach of its commitments under the agreement. If that failed then the Government should take powers at Westminster to allow it to exclude a party in breach, the spokesman added. - (PA)