MPs propping up May government caution against agreement involving different trade regimes for Northern Ireland and rest of UK

DUP MPs have told senior government officials in Downing Street that any Brexit deal giving Northern Ireland a separate customs or trade regime from the rest of the UK would jeopardise the deal that keeps Theresa May in power.

The Conservatives’ minority government is propped up by the DUP’s 10 MPs, in a confidence and supply arrangement that sees them back the Tories in key votes.

But senior DUP figures demanded a meeting on Thursday after reports that the government was preparing to agree that trading relations in areas such as agriculture and energy would remain harmonised between Northern Ireland and the EU after Brexit.

One DUP MP, Sammy Wilson, said publicly that any move to “placate” the Irish government and the EU – by seeking to keep Northern Ireland in the customs union, for example – could see the DUP withdraw its support.

“If there is any hint that, in order to placate Dublin and the EU they’re prepared to have Northern Ireland treated differently than the rest of the UK, then they can’t rely on our vote,” said Wilson. “They have to recognise that if this is about treating Northern Ireland different, or leaving us half in the EU, dragging along regulations which change in Dublin, it’s not on.”

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May was travelling in the Middle East on Thursday but Wilson said he and his colleagues would seek clarification from her in person.

The government has been scrambling to reassure Dublin that it will reach an agreement with the EU27 that will obviate the need for customs controls at the border between Northern Ireland and the Republic.

Downing Street sources insisted the government would not countenance an outcome that involved any part of the UK remaining in the single market or customs union. But the government believes it can still safeguard cross-border cooperation in a series of key areas by agreeing to keep rules and regulations closely aligned.

The DUP leader, Arlene Foster, said last week that any attempt to treat the region differently in terms of Brexit and relationships with the rest of the EU would appear to unionists as an attempt to decouple Northern Ireland from the UK.

Although other DUP MPs privately emphasised they would be seeking a response from the government that would rebuff the border proposals, Wilson is seen as an important gauge of opinion inside the party.

He said the DUP would be “making clear to the government we have a confidence and supply arrangement with them”. That deal allows the DUP to vote against some government policies but guarantees support in crucial votes of no confidence in the Commons and on critical policy measures such as the budget. The arrangement is predicated on a £1bn-plus aid package the DUP extracted from the Tories after the June election.

Wilson’s remarks came as hostility increases between Belfast and Dublin over Brexit, with former DUP leader and first minister Peter Robinson joining in the hostilities saying “the south needs to wind its neck in”.

He accused Ireland of “lecturing the UK” and said it was doing “significant harm to north/south relations”. Earlier on Thursday Ian Paisley, issued a veiled threat to the Irish government warning “if you try to trip us up” it would “reflect badly” in post-Brexit agreements.

His remarks on Irish radio followed his scathing attack on Ireland in Westminster on Wednesday in which he accused the country of acting “disgracefully” over Brexit and of being ungrateful for a £7bn loan during the 2010 International Monetary Fund bailout.

By contrast, the Irish foreign minister, Simon Coveney, told the parliament in Dublin on Thursday that Ireland would not be painted into a corner on Brexit. He said the DUP might hold the balance of power in London but it did not in the republic. He stressed that there could not be “two jurisdictions on this island” after decades of co-operation and peace.

Coveney reiterated that Ireland was willing to block progress of Brexit talks if a form of words offering regulatory convergence was not forthcoming.

The row comes as parliament’s Brexit committee highlighted the challenge of resolving the Irish border problem in a new report on the state of the negotiations. The committee’s chair, Hilary Benn, said: “Our report concludes that we cannot at present see how leaving the customs union and the single market can be reconciled with there being no border or infrastructure. Even by their own admission, the government’s proposals are untested and speculative, so it has yet to set out how no border can in practice be maintained with the UK outside the single market and the customs union.”

The cross-party group of MPs also called on the government to publish a white paper by March next year about its plans for an implementation period after Britain leaves the EU.

“Businesses need certainty and reassurance to stop firms triggering contingency plans which could see activities and jobs move abroad. Ministers assured us that detailed arrangements for the implementation period could be published by March 2018. This deadline must be achieved,” said Benn.

The former Ulster Unionist leader David Trimble also warned that the Brexit border proposals on harmonising trading relations between the region, the Republic of Ireland and the rest of the EU were “extremely dangerous”.

Facebook Twitter Pinterest Theresa May with Arlene Foster in June. Photograph: Carl Court/Getty Images

Lord Trimble, one of the key architects of the Good Friday agreement, said any move to treat Northern Ireland differently from the rest of the UK would clash with the 1998 peace deal, which was based on the key principle that there would be no change to the region’s constitutional status without majority consent.

The former first minister, now a Conservative peer, called on the prime minister to “quash this idea very quickly” as it would start to alarm the unionist electorate in Northern Ireland. Unionist voters might start to believe the Irish government, in pushing this policy through, was trying to break up the UK, Trimble said.

Officials from the UK led by Olly Robbins, and from Michel Barnier’s team, have been engaged in intensive talks as they try to finalise the formal offer May will make in advance of the summit of EU leaders later this month.

The UK hopes it has done enough to persuade the EU27 to move on to the next phase of talks, covering a future trade arrangement. That will require agreement that sufficient progress has been made on three key areas, including the Irish border, as well as the so-called divorce bill, now expected to be about £50bn, and the future status of EU citizens in the UK.