It has been a growing concern – flagged up by police and security services - so there's no real surprise that the threat level to Britain from Irish-related terrorism has been raised from moderate to severe. This means that an attack is a "strong possibility". 

This is the first time the threat level has been published.

The activities of dissident Republican groups, such as the Real and Continuity IRAs, has increased in recent months, and that's what's causing genuine concern.

There is the additional worry that the groups are now working more closely together, pooling cash, weapons and expertise.

Just last week, the head of MI5, Britain's internal security service, warned that the dissident groups, who oppose British rule in Northern Ireland, could strike on the UK mainland.

Jonathan Evans said there had been a "persistent rise" in activity and ambition by dissidents in Northern Ireland over the past three years. 

And while he did not believe the groups could return to the levels of violence wrought by the Provisional IRA in the 1970s, 80s and 90s, this was a "real and rising" security concern.

There is no specific target identified, but there is a desire among the dissident groups to mount an attack on the mainland.

The Real IRA recently warned of potential attacks on financial and banking centres, saying they had helped support Britain's "colonial and capitalist" system. 

'Prestige' attacks

Since the beginning of the year, the dissident groups are thought to be behind around 60 incidents in Northern Ireland.

Hadn't heard about them? Well that's part of the drive to launch operations on mainland Britain.

The Provisional IRA used to claim one bomb in London was worth ten in Belfast in terms of publicity, economic damage and "prestige" for the cause.

We're unlikely to be talking about "no warning" bombs placed in the heart of population centres. 

What we probably will see if the dissidents are successful in mounting an attack is high profile, prestige buildings targeted, with warnings to evacuate the area called through.

But that doesn't always work.

Back in 1998, the Real IRA planned an attack in the town of Omagh in Northern Ireland.

It drove a car bomb into the main shopping area. Warning calls were made but they were unclear and misleading. 

Police inadvertently steered people towards the bomb, rather than away from the danger. 

Twenty-nine people died, 220 were injured. 

It remains the single worst attack in the 30 years of violence which became known as Northern Ireland's "Troubles".

The threat from Irish-related terrorism to Britain is still lower than the overall threat from international terrorism.

It remains unchanged at "severe", which suggests an attack is "highly likely".

Intelligence and security resources, which for 30 years had been focused on combating the threat from across the Irish sea, have in recent years been switched and reinforced to fight the possibility of another al-Qaeda inspired attack like the one on the London transport system in 2005.

Now those resources will be even more stretched as MI5, the police and others try to combat the threat on two fronts.