PEOPLE Before Profit (PBP) has vowed to "block a Tory Brexit" and resist the imposition of a hard border – "whether it comes from London or Brussels".
The pledge is among polices outlined in the leftist party's general election manifesto, which was launched in Belfast yesterday.
PBP, which has one MLA and five councillors, is contesting only two seats in next week's Westminster poll.
The party's sole assembly member Gerry Carroll is its candidate in West Belfast, while Shaun Harkin is running in Foyle.
The manifesto says the party will scrap welfare reform, tackle the climate crisis, fund childcare and end the public sector pay freeze.
On Brexit, it believes people should have the "right to a say on any Brexit deal".
In the 2016 referendum, PBP supported leaving the EU but did not campaign for a Leave vote.
Speaking at the launch, Mr Carroll said: "We are the only socialists standing in this election, to provide an alternative to the big parties who have stood by as our health service was crippled, public sector workers were underpaid and overworked, and who voted for devastating welfare reform measures.
"We know we cannot trust those parties to stand up the Tories – only a socialist can do that."
He said that if elected, the party's MPs would go to Westminster to "vote against Tory cuts, against a Tory Brexit and any no deal scenario, and to stand up for the people of the north who've been failed or underrepresented for far too long".
