Elderly former IRA terrorists are being brought out of 'retirement' to exploit tensions in Northern Ireland over Brexit, The Mail on Sunday has learned.

Sources say the New IRA, formed in 2012, is planning a 'five to six-week campaign of violence and disruption' across Northern Ireland immediately after March 29, when Britain is due to leave the EU.

The Ministry of Defence has put all military bases in the province on red alert. A photograph taken on January 14 at a base next to Belfast International Airport shows a sign warning military personnel they face a 'severe' terror threat level.

MI5 has more than 700 officers stationed in Belfast to combat the threat, The Times has reported.

The New IRA, already regarded as highly dangerous by police and security services, has been bolstered by veterans offering tactical expertise.

Security forces in Northern Ireland have had to deal with two further bomb scares as tensions heightened in the country

A warning sign at a military base in Northern Ireland ahead of the explosion in Londonderry put the threat level at severe. They are now on red alert

They gave up violence after the 1998 Good Friday Agreement, which ended decades of sectarian conflict. But the possibility of a hard border between Northern Ireland and the Republic has tempted them back.

A republican source said: 'These former fighters may be getting on a bit, but they are all seasoned, experienced men who still have a lot to offer the movement and can train, advise, guide and boost morale.'

The New IRA was behind the car bombing a week ago outside a courthouse in Londonderry, an attack aimed at testing new recruits and equipment.

Now it is feared that the terror group – said by one source to be 'awash with cash and guns' – plans to carry out almost daily attacks after March 29.

The remains of the car that was earlier hijacked and packed with explosives before being detonated outside the courthouse in Londonderry, Northern Ireland as fears rise the New IRA is plotting a campaign of disruption after Brexit

The aftermath of a suspected car bomb is seen in Londonderry in Northern Ireland

Forensic officers were called to the scene following the suspected Londonderry bomb attack. Fears are there will be an escalation of violent with the looming Brexit deadline

Hundreds of MI5 officers are being stationed in Belfast to combat the new threat

This newspaper has been told that security officials are warning internally that fortified structures installed to enforce a hard border would become targets, putting the lives of police and customs officers in danger.

A police source said: 'We are expecting the worst and planning for the worst. What we are seeing now, particularly in Londonderry, is that former Provisionals, who supported the Good Friday Agreement, have decided to throw their lot in with the New IRA.

'Our intelligence shows that these are not 'ceasefire soldiers' who joined in the 1990s. These are veteran, experienced Provisional IRA terrorists who were at the front line of the terror campaign in the 1980s. They are experts at manufacturing bombs. They are extremely good at targeting people for murder. They are good at planning operations.

'So these are formidable people we are now having to deal with, both overtly and covertly.'