Media playback is unsupported on your device Media caption Bertie Ahern was prime minister of the Republic of Ireland from 1997-2008

The former Irish prime minister Bertie Ahern has warned dissident Republicans could use a re-introduction of border posts in Northern Ireland to justify their campaign of violence.

He said a hard border, with customs checks, will have to be re-introduced because of the UK's decision to leave most of the customs union under Brexit.

Mr Ahern drew a comparison with the IRA's Border Campaign in the 1950s.

But he said he did not expect a return to large-scale violence.

Mr Ahern was speaking to BBC Newsnight ahead of elections to Northern Ireland's Assembly on Thursday.

The elections were triggered after Sinn Féin, which has shared power with the Democratic Unionist Party for 10 years, withdrew its support from the executive, in a row over a renewable heating scheme.

But there has also been a loss of trust between between Sinn Féin and the DUP over issues such as the legacy of the Troubles and the impact of the Brexit vote.

The UK government has pledged to ensure there will be no return to the "hard border" of the past between Northern Ireland and the Republic of Ireland.

Image copyright Getty Images Image caption There have been protests about possible border checks post-Brexit

But Mr Ahern told Newsnight that the UK's decision to sever its links with most of the EU's customs union means that a hard border, with customs checks, will have to be re-introduced.

"Nobody envisaged that this would end up where it is. And a year ago, I would not say there was a voter that was on the outside that thought of a customs union on the border," he said.

"I tried to warn of that, but people didn't want to listen to it - including the Conservative politicians who should have."

The former prime minister said that the re-introduction of border posts could be used by dissident Irish republicans to justify their existing campaign of violence.

"The 1956-62 border campaign was targeted in a very clear way against checks on the border. There were mainly southern IRA activists who attacked the posts on the border.

"There is no doubt about it. Those people - small as they are - are always dangerous because anyone who plays the game of armed struggle or violence is always the danger.

"They would see checks on the border, and customs officers on the border, and the identification of the border, as in some way justifying the kind of things that they always have in their mind."

The IRA's border campaign, which took place a decade before the start of the modern day Troubles in 1969, involved deadly attacks. But it was on a much smaller scale than the violence that erupted in the late 1960s, prompting the deployment of the British army onto the streets of Northern Ireland.

Mr Ahern's warnings were echoed in stronger terms by Jonathan Powell, Tony Blair's former chief of staff, who was intimately involved in the Northern Ireland peace process during the former prime minister's 10 years in office.

Media playback is unsupported on your device Media caption Jonathan Powell is also concerned about the impact of a 'hard border'

"I believe it will be very dangerous if you start having to have a hard border again," he said.

"If you put in blocks along the border, people are going to be trying to destroy those, and that will create problems.

"If you set up border checks - even if those border checks are 10 miles one side or the other of the border - you're setting people up in static positions. That makes it incredibly easy to shoot at them. And as soon as they come out of those checks, they will be easy to shoot at.

"Now the dissidents are tiny, the dissidents are not really a threat - they're not in any way comparable to the old IRA.

"But it takes very few people to start murdering officers in those circumstances, and once they start murdering them, it's very hard to know how to react."

But Lord Bew, who advised the former Ulster Unionist leader David Trimble, said that a deal may be reached which will ensure that today's so-called "soft border" can be maintained.

"At this moment there's a fear north of the border among the nationalist and Catholic community that you're going to move back towards at least a more visible border, and more irritating checks, and people have become so used to the border being an almost non-existent fact, that this is very irritating.

"It may well be that three years from now we will see that these fears were exaggerated but that's why it's so significant and dangerous that were having this election now."

Media playback is unsupported on your device Media caption George Mitchell says he hopes US president Donald Trump will engage on Northern Ireland

James Brokenshire, the Northern Ireland secretary is likely to convene talks between the main parties to try to re-establish the power-sharing executive after the election.

These are likely to continue during the traditional St Patrick's Day celebrations in the White House on 17 March.

George Mitchell, the former US senator who chaired the Good Friday Agreement negotiations, told Newsnight that Donald Trump should use that occasion to engage in Northern Ireland.

"I hope very much that the president... will focus on the issue and will reassure the guests from both the Republic and Northern Ireland, and other places, that the United States policy of support for the continuing process in Northern Ireland continues - and that we will do whatever we can, recognising the limited nature of our role, to keep that process moving forward."

Nicholas Watt is political editor for BBC Newsnight. Watch his report on the programme at 22:30 GMT on BBC Two - or catch up afterwards on iPlayer