Theresa Villiers has been accused of "avoiding" answering questions on the effect a Brexit would have on our economy.
The Northern Ireland Secretary has declared she will campaign for the UK to leave the EU ahead of the referendum.
But Ms Villiers side-stepped the issue when it was raised in the Commons, with NIO minister Ben Wallace sent out to bat for many EU-related questions.
Shadow Northern Ireland secretary Vernon Coaker raised concerns about the impact of a Brexit as he stressed the importance of the EU to the Northern Ireland economy.
He asked Mr Wallace how he kept "a straight face" as he promoted the Government's position with Ms Villiers sat behind him.
"It is no wonder the Secretary of State is not answering these questions on the economy and avoiding them again," Mr Coaker said. "Can I ask the minister, has he discussed with the ministers of the Executive the survey by the Northern Ireland Chamber of Commerce which showed that 81% of businesses in Northern Ireland support EU membership?
"Isn't it the case that there is little surprise in that finding, given that 60% of Northern Ireland's exports go to the EU, higher than any other part of the UK?"
Mr Wallace replied: "Both myself and my right honourable friend discussed that with the Northern Ireland Chamber of Commerce at a reception on Monday in Northern Ireland.
"If you want to know how I keep a straight face, I look across the despatch box at two members of the Labour Party who are in favour of replacing Trident and remember that they have a leader who has no intention whatsoever of using it or replacing it."
Speaker John Bercow said Trident was not "germane" to the issue of exports.
Mr Coaker replied: "I think we are all amazed by that. That really was going to the bottom of the barrel to try and find something to say on that."
Ms Villiers did face a couple of questions on the EU later during Northern Ireland Questions, but she would not be drawn on her own position.
Labour's Kevin Brennan asked what assessment Ms Villiers' civil servants had made of the impact of a Brexit and whether she was allowed to see it.
Ms Villiers said: "The Government is publishing a number of documents setting out its position on the European Union."
Mr Bercow told off Mr Brennan for being a "very cheeky chappy" because his question was supposed to be on the financial position of the Northern Ireland Executive.
A spokesman for the Secretary of State later rejected the idea that Ms Villiers had avoided any questions. He said: "It is utterly misleading of the Labour Party to claim that the Secretary of State avoided any questions Today.Out of the nine questions asked during the half-hour session, Theresa Villiers answered six.
"Her answers were detailed and covered a full range of Northern Ireland issues, including politics, security and the Government's long-term plan to strengthen the economy."