An SNP leader will accuse the UK of breaching Northern Ireland's Good Friday Agreement over Brexit.

MEP Alyn Smith will echo concerns already expressed by Irish Republicans that the Conservatives have jeopardised the peace process with both their pact with the unionist DUP and their rejection of the single market.

In a keynote debate in Dublin on Thursday will say that current plans to leave the EU will either leave the Irish Republic in defacto control of Northern Ireland or impose a damaging hard border across the island of Ireland.

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He will say: "The actions of the UK government in relying on the DUP for its very existence is to my mind a clear breach of the Good Friday Agreement before Brexit even starts."

Labour has previously expressed concern that the DUP pact jeopardises the peace process. Sinn Fein has argued that the deal is a clear breach. Mr Smith comes close to backing their position.

His remarks - in a debate with an Irish senator at Dublin City University - comes as the Irish Government urged its businesses to brace themselves for Brexit.

Former Irish premier Bertie Ahern savaged Boris Johnson, saying the Tory foreign secretary "couldn't care less" about a border in Ireland.

Mr Ahern said: "After 800 years of conflict, we’ve had close now on 30 years of a very good relationship.

"I think Brexit does nothing to help that. And that’s sad."

Mr Smith, however, will stress the importance of Scotland's relations with both the Republic of Ireland and Northern Ireland and urged common purpose in fighting Brexit.

He said: "The things the UK Ministers are promising - a frictionless border, but a UK out of single market and the Republic within - cannot co-exist in the real world.

"Schrödinger's border is not a credible strategy and the people of this island, and indeed these islands, deserve better.

"The extent to which the Irish aspects of Brexit have not received any serious thought beyond a lamentable 28 pages of contradictory waffle is nothing short of a scandal which any responsible government with a shred of integrity would have resigned over long since.

"We of course have deep concerns over the fact that the devolution settlements in Scotland and Wales are being comprehensively ripped up, but the reality in Northern Ireland is sheer constitutional vandalism.

"The only logical outcomes to any Brexit scenario I can see are either de facto Dublin rule in the North and the border shifts to Stranraer, or a hard border the likes of which you've never seen across the Island of Ireland and the Peace Process ruinously undermined."

Mr Smith will add the EU negotiator Michel Barnier, as a former peace commissioner, "gets" Northern Ireland better than the UK Tories.

Conservatives have long insisted that there are technological means of having a frictionless border in Ireland. However, Brexit secretary David Davis on Wednesday admitted Holyhead, the port in Wales which carries most traffic from Ireland, would have an "extra layer of complexity". Stranraer in Scotland is the second biggest UK port for trade with the island of Ireland.