The Home Office denied ministers had been warned that specific mainland sites, such as Canary Wharf, in London's Docklands area, were being targeted by the breakaway faction of the Provisional IRA, but said a review of security was being carried out.
"There is a review of security under way, but there has been no specific threat from any individual terrorist group that has led to this action," said a Home Office spokesman.
Since the ceasefire, security has been relaxed around likely targets such as the Palace of Westminster, but it is believed that police have reinforced the tight security around the Square Mile of the City of London, a prime economic target for bombers in the past.
The review came as Martin McGuinness, the Sinn Fein chief negotiator, said yesterday: "There are republican rejectionists out there who are obviously opposed to the Sinn Fein peace strategy and who wish to bring it down. These people are unrepresentative and they are totally and absolutely clearly not speaking for the overwhelming majority of republicans who have in election after election supported the Sinn Fein peace strategy."
Calling on David Trimble, the Ulster Unionist First Minister of the new Northern Assembly, to hold his nerve, Mr McGuinness said on BBC radio that there were also unrepresentative loyalist groups attacking the nationalist community on an almost daily basis.
"These can be defeated by combined political action of Unionist and republican leaders working together to implement the Good Friday Agreement."
Tony Blair is prepared to go to Belfast for more talks with the parties to break the deadlock between the Ulster Unionists and Sinn Fein over the establishment of the power-sharing executive, which has threatened to wreck the peace process.
The crisis in the process was discussed in a phone call between Mr Blair and President Bill Clinton on Thursday night. There are rumours at Westminster that during St Patrick's Day celebrations in the United States President Clinton will be exerting pressure on Sinn Fein's leader, Gerry Adams, and Mr McGuinness for the IRA to call a halt to the punishment beatings, which have undermined the process.
Mr Trimble said he was sticking to his demand for a start to the decommissioning of IRA weapons before sitting down in the executive with Mr Adams and Mr McGuinness.
Bertie Ahern, the Irish Prime Minister, ruled out suggestions at Westminster that establishment of the assembly and executive could be suspended until a solution was found.
Meanwhile, in the Republic a rift appeared between Dublin and Sinn Fein over the Good Friday Agreement as four men were jailed yesterday for killing Limerick Garda Jerry McCabe during a 1996 IRA robbery.
During the raid in the tourist village of Adare, Det Garda McCabe, a father of five, died in a hail of bullets. His partner, Garda Ben O'Sullivan, was critically injured. Responsibility was first denied, then admitted by the Provisional IRA.
Mr Ahern, on a visit to Northern Ireland, insisted that the four would serve "whatever sentences were handed down by the court". But Mr McGuinness predicted that the men would qualify for the early-release scheme.
Dublin has repeatedly insisted that Good Friday Agreement early releases were qualified by a protocol, subject of intense argument during final negotiations last April, excluding from the scheme certain prisoners awaiting trial, including the McCabe accused.
The former Brixton escaper Pearse McCauley, 34, of Strabane, and Kevin Walsh, 42, of Patrickswell, Limerick, were jailed for 14 years at Dublin's Special Criminal Court. Jeremiah Sheehy, 36, of Rathkeale, and Michael O'Neill, 48, of Patrickswell, received 12 and 11-year terms respectively. They pleaded guilty to manslaughter. A fifth, John Quinn, aged 30, received six years for conspiracy to rob.
Nora Owen, of the Fine Gael opposition, a former justice minister, said she was "deeply concerned" by the McGuinness intervention and urged the Taoiseach to clarify the position before the Dail.
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