Sinn Féin is celebrating a historic victory in the Stormont assembly elections despite warnings from the Democratic Unionist party that it will block the formation of a new power-sharing executive until the Northern Ireland Brexit protocol is changed. On Saturday evening it was confirmed Sinn Féin would become the largest party, with 27 seats, pushing the DUP into second place. The other big winner was the cross-community Alliance, which increased its first-preference vote share to 13.5%, leapfrogging the struggling UUP and SDLP to become Stormont’s third largest party for the first time. Sinn Féin will nominate its deputy leader, Michelle O’Neill, as the Northern Ireland’s first nationalist first minister. While unionists at some count centres were left visibly stunned by the scale of their defeat, there was jubilation among supporters of Sinn Féin, whose leader, Mary Lou McDonald, told TalkTV she believed a border poll on a united Ireland would be possible “within a five-year timeframe”. But with just two seats between Sinn Féin and the DUP by mid-afternoon, it was not the collapse predicted by the polls. “[The] tumult was not utterly catastrophic for the DUP, but the crown is lost,” said Jon Tonge, a professor of politics at Liverpool University. Alliance looked likely to double its previous total of eight seats, largely at the expense of the UUP, the SDLP, and the Green party, whose leader Clare Bailey lost her seat. It was also a sobering day for Doug Beattie, the UUP leader, who scraped in on the seventh count. Sinn Féin, the DUP and Alliance, and possibly other parties that may clear a threshold, will have up to 24 weeks to form a new executive, under new laws signed off in Westminster. However the DUP leader, Jeffrey Donaldson, said he would refuse to join a new administration until the UK government “dealt with” the protocol in the Brexit deal that put a trade border in the Irish Sea. Without a first minister and deputy first minister, the executive cannot function fully, with ministers prevented from making new policy, signing off budgets or introducing much-needed healthcare reforms. John O’Dowd, a Sinn Féin MLA, urged the DUP to respect the democratic result. Gregory Campbell, a DUP MP, signalled the party could in principle accept O’Neill as first minister. “The people have spoken,” he told RTÉ. Senior DUP sources said they would seek an urgent summit with Downing Street to press home the message that their boycott could put the assembly on pause until Christmas. Under the laws introduced in February aimed at preventing a full collapse of power sharing, parties will have four sets of six-week windows to form an executive, or cabinet. If no executive is formed, the Northern Ireland secretary must call a new election, which in turn must be held within 12 weeks, pushing the chances of a full devolved government back to December. The DUP also faces another urgent dilemma. After a ban on double-jobbing, Donaldson will have to decide if he remains an MP or takes up his new seat as an MLA and force a byelection for Westminster. DUP sources predict he will anoint a stand-in to take his assembly post during the expected Stormont impasse and take the MLA seat himself only if an executive is formed. The surge in seats for the Alliance party reflecting a growing appetite for progressive, non-constitutional politics. “I don’t really like the unionists or the nationalists. I’d like to see a party for Northern Ireland, not for division,” said Natasha Nesbitt, 19, an engineering student and first-time voter. “I feel Northern Ireland is quite behind on issues like abortion. I hope when my generation are older Sinn Féin and the DUP will go down and others will go up.” Colin Lloyd, a retired engineer who switched from unionists to the Alliance party. “With Sinn Féin and the unionists, it’s the same old thing all the time. We are missing out on the bigger issues such as the environment. We are stuck with the same things we had 30 or 40 years ago.” HIs wife, Maureen, a former housing official, said she split her vote among three parties. “It is time to move on,” she said.