The head of MI5 has warned that dissident republicans could attempt to mount a new wave of terrorist attacks in Britain.

The director-general of the British security service, Jonathan Evans, said there had been a 'persistent rise' in 'activity and ambition' by dissident groups in Northern Ireland over the past three years.

Jonathan Evans said that while they did not have the capacity to return to the levels of violence caused by the Provisional IRA at the height of the Troubles, he maintained they represent 'a real and rising security challenge.'

Mr Evans's warning came after the Real IRA publicly threatened to target banks and other financial institutions in London, accusing them of 'financing Britain's colonial and capitalist system'.

The MI5 chief said while MI5's 'main effort' remained focused on international terrorism, it had been necessary to reinforce its presence in Northern Ireland to deal with the heightened threat.

Mr Evans acknowledged that the recent rise in activity by dissident republicans had not been foreseen, having been assumed just three years ago to be 'low and likely to decline further.'

He continued, 'perhaps we were giving insufficient weight to the pattern of history over the last hundred years, which shows that whenever the main body of Irish republicanism has reached a political accommodation and rejoined constitutional politics, a hard-liner rejectionist group would fragment off and continue with the so called armed struggle.'

Since the start of the year there have been more than 30 attacks or attempted attacks on security targets by dissident republicans, compared to just over 20 for the whole of last year.

There are increasing signs of co-operation and co-ordination between the various groups, deploying a greater variety of attack techniques with improved weapons capability - including Semtex explosives associated in the past with the Provisional IRA.

Ultimately, however, he said the dissidents had done little to develop a credible political strategy and many combine terrorism with organised crime, including trafficking drugs.

On the wider al-Qaeda inspired extremist threat, Mr Evans said MI5 was receiving 'several hundred' potential new leads every month, while at any one time there was likely to be a 'handful' of investigations under way into plots believed to involve the 'real possibility' of an attack.