When I negotiated the Good Friday Agreement nearly 20 years ago, no one foresaw a day when the -United Kingdom would be leaving the European Union. It was impossible to imagine how the issue of the border between Northern Ireland and the Irish Republic, from which the barriers were removed as part of the agreement, would again become an issue of such political importance.

We have the Taoiseach, Leo Varadkar, threatening to veto the Brexit negotiations unless Theresa May gives a formal written guarantee that there will be no hard -border, and we keep hearing the argument that a departure of the UK from the single market and the customs union would put at risk the peace process and Good Friday Agreement. In other words, if the -border gates go back up again we will be back into the Troubles.

This cannot go unchallenged. The reason the issue of the border has been brought up in the way it has is not because of any practical reasons but because of the internal -politics of the Irish Republic. The Taoiseach has been in desperate negotiations with other party leaders in order to prevent a general -election being triggered. He is -snarling at London, trying to make a big issue about the border, because he is worried Sinn Fein might benefit if he does not.

It is not true that Brexit in any way threatens the peace process. There is -nothing in the Good Friday Agreement which even touches on the normal conduct of business between Northern Ireland and the Republic. Leaving the EU does not affect the agreement because the EU had nothing to do with it — except that Michel Barnier turned up at the last moment for a photo opportunity. The EU does have a peace and reconciliation programme for Northern Ireland but there is no provision for it in the EU budget. It is financed from loose change in the drawer of the European Commission.

What would threaten the peace process, on the other hand, is Dublin’s suggestion that the border question could be solved by the UK having an internal -border running down the Irish Sea, with mainland Britain leaving the single market and the customs union and Northern -Ireland remaining within them.

The Belfast Agreement recognises British sovereignty in Northern Ireland, and recognises Northern Ireland as part of the UK. To have provisions treating us as if we are not part of the UK is clearly contrary to that agreement and is something no unionist is going to support.

Once it begins to dawn on the unionist electorate that the Irish government is trying to break up the UK then we are into very dangerous territory indeed. The government needs to quash this idea very quickly, and make it clear that we will not have any damage done to our constitution.

I do not want to go back to the days of fences and barriers any more than does anyone else. But then neither does Theresa May. The government has repeatedly made it clear that it has no intention of putting up border posts on its side of the border. But it can’t give assurances that the EU won’t insist on border checks on the southern side. That will be an external EU border, and it will be the EU’s decision.

There is no reason from our point of view why the border cannot remain open. We already have to deal with the issue of -smuggling because, in spite of the single market, there remain differences in -regulations and duties. When I was first minister there was a big problem with smuggling diesel. I brought this up with Tony Blair, but I -suspect it was allowed to go on partly because former leading members of the IRA were deeply involved in it.

Then there is the issue of migration. A couple of years ago the Irish police stopped a vehicle that had come over the -border from Northern Ireland into the Republic carrying half a dozen people who were -travelling to work, but had no right to work there. They sent them straight back again. The border has never gone away entirely. There is no reason it can’t continue to be policed without hard barriers, even after Brexit.

It is illogical for the EU to demand an agreement on the border before it will even discuss a trade agreement. We don’t know what the terms of the trade agreement will be, so how could anyone know what kind of a border would be needed? We might end up with an agreement where we don’t have tariffs — in which case the whole issue would ease considerably.

The real reason why the border has become such an issue is that Sinn Fein is -trying to exploit Brexit to break up the UK. And the whole reason Sinn Fein collapsed in Northern Ireland’s -assembly is because its leaders realised that if they were serving in British institutions — and the Northern Ireland Assembly is a British institution — it would be much -harder for them to do this.

What Leo Varadkar is doing is trying to appeal to Sinn Fein voters. He hasn’t learned the lesson that some Irish nationals have painfully learned in Northern Ireland: that you can’t out-Sinn Fein Sinn Fein. All he is doing is validating its position. For its own reasons, the EU is egging him on.

It just shows you how desperate the EU and Irish nationalists are that they’re -clutching at these straws.

To treat Northern Ireland as if we are not part of the United Kingdom is something no unionist will support

David Trimble was First Minister of Northern Ireland from 1998 to 2002 and shared the Nobel Peace Prize for his work on the Good Friday Agreement.