The Prime Minister has backed Northern Ireland Secretary Theresa Villiers after her anti-EU stance prompted calls for her resignation.
Nationalist and republican politicians questioned her position after her decision to support the campaign for the UK to leave the EU.
Martin McGuinness called for her to resign, saying her position showed she was “cut off from public opinion”.
As Ms Villiers sat nearby in the chamber on Monday, David Cameron told the House of Commons: “The Secretary of State for Northern Ireland does an excellent job.
“She is exercising her ability to reach a personal decision and to campaign for Britain to leave the EU and that’s absolutely right she is able to do that.
“I think the key thing is everybody in Northern Ireland should make up their own mind based on the evidence and I look forward to coming to try to help persuade them to remain in a reformed EU.”
Earlier in the month, Mr Cameron had indicated to the Commons that he plans to visit Northern Ireland ahead of the EU referendum.
Theresa Villiers was among a string of cabinet members – including Michael Gove and Iain Duncan Smith – who have decided to split from the him in recent days and call for the UK to leave the EU.
Nationalists in Northern Ireland had criticised her stance.
Fianna Fail, a republican party which commands a large vote in the Republic but which has no discernible presence in Northern Ireland, also issued a statement.
It said that Ms Villiers’ views should be respected, as should the “sovereign right of the British people to decide their future”.
But the statement from its foreign affairs spokesman Brendan Smith also said: “Brexit would be felt most keenly here in the Republic of Ireland, the only state with a land border to the UK and our biggest trading partner.
“A UK exit from the EU would create a new wall of regulations from Derry to Down and undo much of the important work on cross-border bodies over the past 18 years.
“In light of this, the fact that the Secretary of State for Northern Ireland would campaign for an exit is deeply worrying.”
She was also quoted on the BBC Northern Ireland website on Monday as saying: “We give, at the moment – £19bn a year to the EU and so we could actually, if we left, we could still afford to fund every single one of those programmes and still have significant sums left over for other priorities in Northern Ireland.”