Lord (David) Trimble, the former Ulster Unionist Party leader, in the News Letter Belfast office for an interview in the aftermath of the 20th anniversary of the Belfast Agreement. Pic by Ben Lowry

The former Ulster Unionist Party leader said he believes that the republican party are hoping for a surge of support caused by the UK’s departure from the European Union.

However, he thinks that their plan will fail.

Speaking to the News Letter in the aftermath of the 20th anniversary of the Belfast Agreement, Lord Trimble, formerly David Trimble, said: “The underlying reason for Sinn Fein pulling out wasn’t the reasons given.

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Ulster Unionist Party leader David Trimble and Sinn Fein President, Gerry Adams pass within touching distance outside Castle Buildings, Stormont during a break in the 1998 negotiations before the signing of the Good Friday Agreement. Picture Pacemaker

“The underlying reason was Brexit, where they’re hoping that if they run a strong campaign on the Brexit issue that it might increase their support, give them another surge in terms of popular support which might help them in the Republic where there will be an election soon.”

He adds: “So, they’re hoping that they get a surge from the Brexit issue and that it might put them in a situation where they can start to call for a border poll.

“You could say that what Sinn Fein have done is that they have given up on sharing power with the Northern Ireland parties and therefore they are just envisaging a united Ireland.”

He adds: “It’s not going to work for them because thankfully we have, in the Belfast Agreement, some basic rules about when you can have a border poll.

Ulster Unionist Party leader David Trimble and Sinn Fein President, Gerry Adams pass within touching distance outside Castle Buildings, Stormont during a break in the 1998 negotiations before the signing of the Good Friday Agreement. Picture Pacemaker

“It’s not like Scotland where there is nothing in the Scottish legislation and Alex [Salmond] was able to run rings around [David] Cameron in their negotiations — and we were very lucky not to have a bad result there.

“With regard to Northern Ireland, the secretary of state can only act if he is satisfied that there are reasonable grounds that there may be a desire for a change. He can’t actually have a referendum because he thinks politically it may be desirable. He’d be judicially reviewed and that would go down.”

Lord Trimble says it would be “a good precautionary move” for unionists to put together the argument that says a border poll is not going to produce its aims.

He also thinks that what he believes to be Sinn Fein’s strategy will fail because Brexit negotiations “are going to come to a conclusion sooner rather than later”.

Asked why he thinks that, he replies: “We’ve reached a point in the negotiations where quite a number of things have been sorted out in principle, and there’s really just a couple of issues to deal with.

“... It reminds me of the period just before the last week of the [1998 Belfast] talks … and once the principals, and by that I mean the heads of government at a serious level, once they start to look at this, I think you’ll find things accelerating so that Brexit may be off the table.”

But the Irish border is one of the unresolved issues. What does Lord Trimble think about that problem?

“The big part of the border decision is ... what is the position on tariffs? ... [Michel] Barnier had said things which imply that there will be no change in tariffs, that the EU will just continue to have its existing tariff arrangements.”

Lord Trimble adds: “From an economist’s point of view, free trade is always the best policy, but bureaucrats, particularly those who are looking at the money they raise through these things, will be reluctant to go down that line, particularly as they’re about to lose 12% of their revenue, through the UK leaving.”

If the existing EU arrangements on tariffs remain it will be “very bad news for Dublin”.

“The existing EU legislation, as I understand it, puts on the country who has part of the EU’s external border, an obligation on them to put in place appropriate arrangements for enforcing the tariff.

“ [Ireland will] be under an obligation to put the appropriate infrastructure on their side of the border ... whereupon various republicans will turn up and burn it down, and all the rest of it, but they’ll be burning down Irish infrastructure because we’re not going to put anything on our side of the border.”

What about the risk of Britain suddenly agreeing that Northern Ireland stays in the customs union and single market, ie a border in the Irish Sea?

“Well, I would discount that risk because the prime minister has been absolutely clear, time and time again about her support for the Union and her opposition to having a border drawn down the Irish Sea.”

But what if it did happen? “It’s not going to happen and I don’t want to get into a long discussion about just how bad a thing that would be, but that would be a bad thing, a very seriously bad thing to do.”

• In Monday’s paper, Trimble on the Irish language

When asked if he thinks the London coalition government will survive, Lord Trimble replies: “Oh yes, oh yes.”

Pressed about the possibility that enough of the core of pro-Remain Tory MPs break away for the government to fall, he says: “There might be a few kamikazes, but there will only be a few, and it’s not going to get them anywhere.”

But could the government lose in a House of Commons vote on the UK as a whole staying in the customs union? This interview with Lord Trimble was carried out before the government lost on that issue in the House of Lords, so his answer has to be seen in that context.

He said: “There is the semantics of Labour, who talk about ‘a customs union’, not ‘the customs union’ ... look, the customs union, and the single market, and there’s a third element — they’re all part of the same thing. Separating them out is just nonsense and Labour’s official position is that they want to improve the government’s Brexit proposals, but they are not going to wreck it.”

Lord Trimble believes that when Sinn Fein see that they are not getting the surge in support that would put pressure on London to have a border poll, “they’ll have no option then but to troop back into the NI Assembly”.

“At which point,” he adds, “to get them trooping back in, it’ll be necessary — and I know it’s irritating, the situation where they’ve gone off, caused us all an awful lot of anxiety — but just for the sake of trying to get things running smoothly, we’ll have to give them a few wee, you know, what Seamus Mallon once referred to ‘get the goodie bag out’.”

But will that not feed a long term trend in which SF know they can cause crises and get reward, even if small reward?

Lord Trimble does not answer this point directly but instead talks about a situation in which there is no executive.

“Why should that veto the existence of the assembly? You can have an assembly without an executive and the precedent on this is Wales.”

In the Cardiff parliament’s first few years, he says, there was an assembly but no executive.

“It’s not as efficient as having an executive because you have a slower decision-making process, but it’s preferable to not having an executive or an assembly ... I’m told that the Welsh model is still something that the government thinks up from time to time.”