Trade worth €65 billion between these islands as well as hundreds of thousands of jobs, depend on the UK having “unfettered access” to the single market and customs union, a conference in Dublin was told.

The comments came as the question of the Irish Border dominated Brexit negotiations in Brussels on Friday.

Addressing the conference, “Ireland and the Future of Europe”, Welsh cabinet secretary for finance Mark Drakeford said “at every opportunity” the Welsh government was pushing “the importance of full and unfettered access to the single market”.

He told the conference that 70 per cent of Ireland’s trade with the rest of the EU passes through Holyhead in north Wales, and the parliament there was not willing to roll back the integration of seamless trade which had been developed in recent decades. He said he wanted to see “valuable migration” continue as well as academic partnerships such as Erasmus and Horizon 2020 and the EU-funded, Ireland-Wales Interreg programme.

Mr Drakeford said, “Theresa May committed to continued participation in such programmes in her Florence speech and we expect the UK government to hold good to this”.

“These positions have made clear our commitment to maintaining full and unfettered access to the EU’s single market and to remaining in the customs union.

“A fully autonomous trade policy in all likelihood would mean leaving the customs union with the EU” and this would likely lead to trade barriers which would “change the nature of Welsh ports,” he said.

Mr Drakeford said Brexit “will happen”, adding that “there is no going back to 1972”. He said UK constitutional arrangements would have to be reworked to transfer power from the Westminster parliament to a council of ministers including the Welsh, Scottish and Northern Ireland governments.

Scottish negotiator

Mr Drakeford was supported by the Scottish parliament’s Brexit negotiator Mike Russell, who said the Scottish and the Welsh were at one on the issue of remaining in the single market and customs union. He said Scotland wanted to remain in the EU and “our only compromise – not as good as membership – is remaining in the single market and customs union”.

Former Fine Gael leader Alan Dukes said there appeared to be agreement on no tariffs. He said there was “only one circumstance in which that can be achieved, that is for the single market to apply to the whole of the UK”.

Ulster Unionist Steve Aiken said the value of trade between these islands was €65 billion and involved “hundreds of thousands of jobs”. He said the UK would have to pay a bill to leave the EU and would have to continue to have access to the single market to maintain that trade and jobs. He said they “should get on with it”.