The Labour Party has changed its rules to allow people in Northern Ireland to become members.
Trade unionist Andy McGivern challenged Labour rules
Delegates at the party's annual conference in Bournemouth voted on a rule change on Tuesday.
More than 86% of delegates voted in favour of changing the party's rules, it was announced on Wednesday.
For decades, Labour sympathisers have campaigned to change membership rules.
However, the party resisted such change, citing its links with the SDLP as grounds for excluding would-be members from Northern Ireland.
But Belfast GMB trade unionist Andy McGivern mounted a legal challenge against the rules, backed by the Commission for Racial Equality.
He said the ban was racist, claiming it discriminated against people from the province.
The party's own legal advice was that they had a weak defence if the matter went to court.
Mr McGivern will now put his legal case on hold, but may re-activate it if he does not believe Labour is offering real membership to people in Northern Ireland.
Although some trade unionists and MPs remain opposed to changing the rules, the proposal won the backing of the party's national executive committee in July before being passed at its annual conference.
The party says although it will now accept membership applications in the province, it has no immediate plans to contest elections in Northern Ireland.
Mr McGivern said the result was a "major leap forward" for politics in Northern Ireland.
"All I ever wanted was to be a member of the party I have always supported and for people in Northern Ireland to be given an alternative to the polarised politics they have been subjected to," he said.
"However I am not going to get carried away. This is the first step.
"I will now wait to see whether this means Labour will set up constituency parties in Northern Ireland and I will consult with my legal team about where we go.
"If we are not given the same membership rights as people in England, Wales and Scotland then that means all that we are being offered is a watered-down form of membership. That would be unacceptable."
SDLP chairman Alex Attwood insisted it would not have any effect on his party.
"In fact, it is most likely that the membership of a British political party will appeal to those of a unionist outlook and not the nationalist community nor the values that the SDLP stands for," he said.
Monica McWilliams of the cross-community Women's Coalition said the impact of the decision could only be positive for Northern Ireland politics.
"Labour is a firmly pro-agreement party, and if local membership entices more people to get involved in politics, this will be good," said the former South Belfast Assembly member.
"It will also help to focus minds on issues that are beyond traditional Northern Ireland politics.
Northern Ireland-born Labour MP Kate Hoey said it was a "tremendous result".
"We will see how many in Northern Ireland want to join, but I would say to people there that the more people who join, the more chance Northern Ireland has to make its voice heard in government," she said.
Democratic Unionist MP Gregory Campbell said his party welcomed "any move which reaffirms Northern Ireland's position within the British political system".
Northern Ireland Conservative Party chairman Julian Robertson said Labour had made "progress towards giving Northern Ireland residents access to the same democratic opportunities that are taken for granted elsewhere in the United Kingdom".
Boyd Black of the Labour in Northern Ireland movement said pressure on the party had paid off.
"This is a basic democratic right to join the party that governs us," he said.