Theresa May will receive a very clear message that Northern Ireland opposes Brexit when she visits before triggering Article 50, Sinn Fein has said.
The party's northern leader Michelle O'Neill said she intended to meet the Prime Minister to voice her strong opposition.
Mrs May is expected in the region later this week ahead of formally starting the process of the UK exiting the EU next Wednesday.
Her visit will coincide with the final week of crunch talks to form a new powersharing executive at Stormont.
In Northern Ireland, 56% voted to remain in the EU in last year's referendum.
A Sinn Fein demand for the region to retain special EU-designated status post-Brexit has been rejected by the Government.
"The British Government are acting against the interest and the expressed wishes of a cross-community group of people here that want to stay in Europe," said Mrs O'Neill.
"We will be making that message clear to Theresa May when she is here - apparently she is coming later in the week - we will make this message very clear to her, that this is not good, this is bad news for the people of Ireland."
If Mrs May meets the main political leaders in Belfast, she will hear a different message from the Democratic Unionists. The region's largest unionist party campaigned for Brexit and has insisted the referendum was a UK-wide vote.
Sinn Fein deputy president Mary Lou McDonald said there was an obligation on Irish Taoiseach Enda Kenny to represent the interests of the entire island and make clear to Mrs May that the wishes of the electorate north of the border had to be respected.
She said Mr Kenny had to "take a stand in the national interest".
"This business of dividing Ireland north and south is over now with the advent of Brexit," she said.
"We now all hang together or we will surely hang separately.
"All of our interests are absolutely intertwined, we will be relying on each other to stand up for each other and to protect each other and, in a very special and particular way, that places a big, big obligation on the Taoiseach."
If negotiations on restoring powersharing fail to reach consensus by next Monday, Northern Ireland could be facing another snap election.
The last election, at the start of the month, was triggered by the collapse of powersharing amid a row between the DUP and Sinn Fein over a botched green energy scheme. The subsequent campaign laid bare a raft of other disputes dividing the main parties.
Mrs O'Neill said the week ahead would be "crucial" for the fate of powersharing. She insisted Sinn Fein would not accept a return to direct rule from Westminster if negotiations fail.
The Sinn Fein leader reiterated her belief that blame for the logjam lays with the UK Government, which she accused of failing to honour previous agreements.