It is not right to have an Irish border poll at this stage, the prime minister has said.
But Theresa May told MPs that Northern Ireland's political parties should focus on talks to restore Stormont.
Nigel Dodds of the Democratic Unionist Party said a poll would be "divisive".
He argued that Sinn Féin's demand for a referendum on Irish unity "as soon as possible" fell outside the terms of the 1998 Good Friday Agreement.
Sinn Féin's Michelle O'Neill said a vote was justified because the UK's withdrawal from the EU would be "a disaster for the people of Ireland".
But speaking in the House of Commons on Tuesday, Mrs May said: "What we should all be focusing on is bringing the parties together to ensure that we can continue to see the devolved administration in Northern Ireland working in the interests of the people."
Mr Dodds said the prime minister had been clear that the "circumstances have not been met" for a vote on the Irish border.
"The collapse of devolution by Sinn Féin has caused enough uncertainty and division in Northern Ireland without that being further compounded through a border poll," he added.
Talks between the Stormont parties are continuing with a view to restoring a power-sharing Northern Ireland Executive.
The negotiations are taking place after the Northern Ireland Assembly election earlier this month, which saw an end to the unionist majority at Stormont.
Sinn Féin now holds just one seat fewer than the DUP.
The parties are halfway through the three-week period they have to form a new executive.
Another election can be called if the talks fail, and ultimately power could return to the UK parliament at Westminster.
Social Democratic and Labour Party (SDLP) leader Colum Eastwood said it was "unfortunate" that a round table meeting involving all the parties was cancelled at the last minute.
He said the decision had possibly been taken "at the behest of the DUP".
He told reporters at Stormont that it would have been the first time during the current process that all the parties had met together.