Such a high level of threat warning has not been seen here for a number of years. An increased number of police officers were visibly back on the streets after the PSNI said it has stepped up its security operations across the province.

Just days after it was announced that the Queen and the Duke of Edinburgh are due to visit Northern Ireland next month officers have been forced to increase activity on the ground and set up vehicle checkpoints across the province after receiving intelligence that the threat from dissident republicans has intensified.

One of the first checkpoints to be set up today was at the Foyle Bridge in Londonderry this morning.

The alert has sparked concern of a return to the "bad days of the Troubles".

Police today said they would urge the public to be patient and co-operate with them in the operation "which is designed to ensure the safety of the whole community" and to report any suspicious activity.

Although this is the first time in a number of years the police have issued such a high level warning the threat from dissident republicans has been bubbling under the surface for several months.

Two weeks ago the PSNI warned that dissidents were preparing a series of firebomb attacks across the province and urged business owners, including retail parks, to check their premises for devices and look out for anything suspicious.

And last November, Chief Constable Sir Hugh Orde said he believed the threat against his officers was the highest it had ever been during his service after the Real IRA shot and wounded off-duty policemen.

An officer in his early thirties was hit a number of times after a gunman opened fire on his car in Dungannon. He managed to drive to Dungannon police station where colleagues gave first aid and he was taken to hospital.

In Londonderry the Real IRA shot a 43-year-old Catholic police officer as he dropped his child off at school. The officer suffered injuries to his face and arm.

Following the shootings Sir Hugh said: "I am worried, my officers are worried, but that will not deter us."

It is now feared dissidents are planning another shooting or bombing.

Further details of the terrorist threat were due to be discussed at tomorrow's meeting of the Policing Board.

UUP Policing Board member Basil McCrea MLA said he hoped the province was not about to see a "return to the bad days."

"The PSNI will not have taken this decision lightly to warn the public and step up operations. We are expected to get a proper briefing on the matter at tomorrow's Policing Board meeting."

Mr McCrea added: "This is a very regrettable state but the big difference between now and during the Troubles is that the police are engaging with the community and I believe they will get a lot of support. We do not want a return to the Troubles. These dissidents do not have community support. We must not allow them to drag us back."

A Sinn Fein spokesman added: "These groups have no strategy and little to offer. They have no support within republican communities."