Critics say Northern Ireland secretary’s cabinet position would be untenable, given potential harm Brexit could do to province

The Northern Ireland secretary, Theresa Villiers, has been called upon to quit her cabinet post if she backs the campaign to leave the EU, amid warnings that a UK exit would put the peace process at risk.

Villiers is one of three cabinet ministers said to be ready to campaign for the UK to leave the 28-nation bloc when David Cameron names the date for an in/out referendum, which is expected later this year.

As leading Tories prepare to reveal which side they will support, there is a growing belief that Villiers’ position in the cabinet will become untenable over fears an EU exit could stoke sectarian tensions and do severe economic damage.

On Saturday, the only Irish nationalist party represented in the House of Commons, the Social Democratic and Labour party (SDLP), joined forces with the Liberal Democrats to question whether Villiers could remain in her post if she campaigns for Brexit.

Colum Eastwood, the newly elected SDLP leader, said she could not act in the interests of Northern Ireland if she wanted the UK to leave the EU. He also argued that Britain’s departure could put the province’s £1.6bn trade with the Irish Republic at risk.

“While Theresa Villiers is obviously entitled to engage in the internal and long running Tory battles over Europe, her role as secretary of state for Northern Ireland places upon her a separate responsibility,” he said.

“That role should require her to represent the best interests of people in the North. A Brexit is not in our interest. It is not in the interest of our economy or in the interest of our society.

“All the evidence, all the major voices in our agricultural and business communities, have warned that a Brexit would devastate the fundamentals of our economy.”

Eastwood pointed to an Economic and Social Research Ireland report at the end of 2015 which said there would be “very serious consequences” for the island as a whole, but particularly Northern Ireland. Northern Ireland has received €1.3bn (£950m) of EU since 1995 to fundpeace programmes aimed at promoting cohesion between communities and improving stability.

Cameron announced last week that ministers, including those in the cabinet, would be able to campaign against the official government line on the referendum, after he was told that some ministers could resign if forced to take a pro-EU line.

The Liberal Democrats’ spokeswoman for the EU referendum campaign, the MEP Catherine Bearder, said Villiers should resign if she backed a British exit. “Given the disastrous impact Brexit could have on the Northern Ireland peace process, it would be highly inappropriate for Theresa Villiers to remain in her post while campaigning to leave the EU,” she said.

“Leaving Europe would risk stoking sectarian tensions and undoing years of peacebuilding, much of it funded through EU peace programmes. It would also fundamentally transform the UK’s relationship with the Republic of Ireland and put at risk the open land border we currently share.

“David Cameron must stop putting the interests of his party ahead of those of the country. Government ministers should not be able to campaign for an EU exit if this completely goes against their role and responsibilities.”

The Irish government also reiterated that a Brexit could damage Anglo-Irish trade and reverse the gains made during the Northern Ireland peace process.

An Irish government spokesperson said: “It is a matter of strategic importance. The UK’s continued membership of the EU is hugely important to Ireland’s own interests, to the wider bilateral relationship, and to the interests of the EU as a whole.”

Eastwood added that a UK departure from the EU would leave Northern Ireland isolated economically and socially from the rest of the island, which would remain in Europe. The SDLP holds three Westminster seats – Foyle, South Down and South Belfast.

Both the SDLP and its larger nationalist rival, Sinn Féin will call on their electorates to vote against a Brexit in the referendum.

The position of the two main pro-union parties, the Democratic Unionists (DUP) and the Ulster Unionists, remains unclear, but some influential unionist politicians have indicated they would support a vote to leave the EU.

The former Northern Ireland finance minister and DUP MP for East Antrim Sammy Wilson shared a platform last November with the Ukip leader, Nigel Farage, during a Leave The EU event in Belfast.

One of the major factors in holding the DUP back from supporting an exit from the EU is the politically influential Protestant farming community, which has done relatively well out of European agricultural subsidies.

The Northern Ireland office did not respond to requests for comment.