MI5’s joint terrorism analysis centre has circulated a warning raising its assessment of the risk of an IRA attack to “significant” — just one level below the threat from Al-Qaeda. The warning means the risk of an IRA attack against London or other targets is the highest since the 1997 ceasefire.

The move reflects the spy service’s latest assessment of the fallout from the £26.5m Belfast bank robbery in December, in which the IRA has been implicated. New intelligence also indicates the IRA is updating its list of potential targets on the mainland.

The MI5 warning comes as Sir Ian Blair, the Metropolitan police commissioner, warned last week that his force is on standby for a terrorist attack, possibly during the run-up to the royal wedding or the general election.

Officials said Andy Hayman, the assistant commissioner and Scotland Yard’s counterterrorism supremo, was taking a close interest in the IRA threat.

A senior Home Office official said: “Following the demise of the peace process in December and the statement by the IRA earlier this month, we have as a result to take increasingly seriously the possibility of IRA attacks both here and in Northern Ireland.”

In its statement, the IRA made plain its anger at being accused of organising the bank raid. “We do not intend to remain quiescent within this unacceptable and unstable situation. It has tried our patience to the limit,” it said.

While some security experts saw the statement as “sabre-rattling”, others said it was a warning that the IRA was prepared to resort to terrorism if it was not satisfied with concessions offered during the peace process.

Following the new risk assessment, dozens of senior figures including MPs, judges and security officials in Northern Ireland and on the mainland have been warned to be extra vigilant about the possibility of an IRA assassination.

Government officials say there is now a “level of uncertainty” that is prompting fears that the IRA could decide to return to a bombing campaign.

Throughout the peace process, the IRA’s intelligence department has been keeping its weapons stock up to date and reviewing its information on potential terror targets on the mainland and in Northern Ireland.

It has also maintained its training facilities, most of which are in remote parts of the Irish republic. oIn a further development in the laundering of the Northern Bank money, Irish police have arrested a 56-year-old woman they claim is “the central, key figure”. She has been on numerous trips to Bulgaria, allegedly to arrange movement of IRA money.

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