Leaving the customs union makes a hard border more likely at the Irish land frontier, above, say critics of the UK decision to leave the union. Picture by Arthur Allison: Pacemaker Press

Downing Street moved to state “categorically” that the UK will be leaving the customs union as Theresa May and David Davis prepared to hold talks with Brussels’ chief Brexit negotiator Michel Barnier.

Confirmation of the government’s approach may placate Tory MPs and ministers who are keen for a Brexit arrangement which allows the UK to strike trade deals around the world – something which a customs union could have prevented.

The MP for East Antrim, Sammy Wilson MP, said: “Hopefully this will now silence those within the cabinet who are conducting a daily guerrilla warfare against the referendum result and the stated aims of the government that the UK will be making a clean break with the EU.”

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Mr Wilson, who is the DUP’s Brexit spokesperson, added: “The statements from the chancellor and the home secretary have been highly damaging in the run up to negotiations on the withdrawal arrangements and only weaken the prime minister’s hand in those negotiations.”

He added: “There are those who try to use Northern Ireland as a reason why we should not be leaving the customs union. Ironically many of those who now claim to be experts on the impact of leaving the EU on this part of the UK for years never had any interest in our country.

“Even worse some of those in the Labour party and the SNP, who now agonise about a hard border gave support to the IRA whose campaign resulted in checkpoints on every main road, watchtowers, army patrols and cameras all around the border yet now they claim to be concerned about a few checks on cross border trade.”

The Green Party leader Steven Agnew MLA said: “If the UK government is earnest in its insistence that there will be no hard border on the island of Ireland, then there is no getting around it – we must remain in the customs union.”

Stephen Farry MLA, Alliance deputy leader, said: “This announcement poses major implications for the avoidance of a hard border on the island of Ireland, as well as the implementation of the EU/UK joint report from phase one of the Brexit negotiations.

Mr Farry, who is Alliance’s Brexit spokesperson, added: “A hard border can only be avoided if the UK as a whole or Northern Ireland as a specific region is part of the same customs arrangement as the rest of the EU.

“It must be clearly understood continued participation in the single market, either by the UK or specifically Northern Ireland, as Alliance has proposed, is essential to not only avoiding a hard border but also to protect north-south co-operation and the all-island economy.”

The SDLP Brexit spokesperson Claire Hanna MLA said: “The narrow, isolationist, and nativist agenda which appears to have gripped the Conservative Party is incompatible with securing a deal which defends the economy and interests of people in Northern Ireland and across these islands.