Dissident republicans determined to make a "big show" in the near future are plotting gun or bomb attacks on mainland Britain, according to a former IRA commander who has turned informant.

"As we are speaking, there are people somewhere in Ireland thinking about how they can bomb England," Sean O'Callaghan told The Independent on Sunday. "There is no doubt in my mind that there will be serious attempts at an attack."

With the centenary of the Easter Rising, which marked the birth of modern Irish republicanism, in three years' time, we are in a "very dangerous period", according to Mr O'Callaghan, who now lives in England for his own safety.

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"There's an inevitable momentum as you move towards 2016, as dissidents become more determined to be relevant – they'll be very keen to prove they are the big show in town on the republican side."

Speaking after a presentation on the dissident threat to experts at the International Centre for the Study of Radicalisation (ICSR), King's College London, last week, he added: "If they can undertake operations here, they will."

The dissidents are "trying to kill people all the time", claimed the 59-year-old former member of Sinn Fein's national executive and a veteran of more than 60 terrorist attacks in the 1970s.

Once the head of the IRA's southern command, Mr O'Callaghan turned himself over to the British authorities in 1988. He served eight years in prison for two murders before being granted early release by the Queen.

His warning comes amid raised tensions in Northern Ireland, after weeks of rioting provoked by a decision to stop routinely flying the Union flag on Belfast City Hall.

Commenting on Mr O'Callaghan's claims, Dr John Bew, ICSR director, said: "There's a very small prospect of any return to the Troubles. However, do these people pose a serious threat to life? Absolutely – there's no question about that. Are they trying to kill police officers every day? Yes." He added: "The fantasy of any dissident is damage in London, because it is ultimately about British occupation of Ireland. It is not an empty threat, but they do not have the same network or sympathetic diaspora that the Provos had."

In 2012, dissident republicans were responsible for more than 80 gun and bomb attacks in Northern Ireland and are a "potent threat", according to Terry Spence, chairman of the Police Federation of Northern Ireland (PFNI). He agrees with Mr O'Callaghan, saying that a switch to attacks on the mainland "cannot and should not be ruled out".

Mr Spence added: "There's been a significant improvement in the situation in Northern Ireland since the signing of the Good Friday Agreement. However, I cannot remember the police facing a more difficult period since the inception of the PFNI, with diminishing resources and the dissident threat coupled with loyalist paramilitaries, who seem hell-bent on stirring things up."

If dissidents succeed in provoking attacks by loyalists, "that could develop into a serious threat to the institutions in Northern Ireland," Mr O'Callaghan warned. "It's not as bad as it was, but be complacent at your bloody peril."