Plaid Cymru have won 42 seats in Wales’s Senedd election, putting the Welsh nationalists in position to form a minority government and ending more than 100 years of Labour hegemony.

Polls consistently suggested Plaid Cymru and Reform UK were neck and neck in the race to become the biggest party under Wales’s new more proportional voting system. As in last year’s closely watched Caerphilly Senedd byelection, however, the contest was not as close as predicted. Reform has come in second, with 34 seats – up from 1% of the vote share in 2021’s election.

Labour, for so long Wales’s political behemoth, has limped into third place with just nine seats in a 96-seat parliament.

Victory for Plaid makes a Welsh independence referendum a future possibility, and means all three of the UK’s Celtic nations would be controlled by separatist parties, raising the prospect of significant constitutional disputes with Downing Street.

Rhun ap Iorwerth, the Plaid leader, had looked calm and confident as he arrived for his count at Venue Cymru in the north Wales seaside town of Llandudno, where the party won three seats, Reform two and the Tories one.

In a speech after securing his seat, ap Iorwerth said it had been a privilege to serve the community he had been raised in – Ynys Môn, the island of Anglesey. He continued: “Today is about the future of our communities here and our nation as a whole. We have offered leadership locally as we offer leadership to all of Wales. As the story of this election has emerged today, it has become clear that Wales has demanded change of leadership.

“My sense of service to my community and my belief in our nation drives me every single day and Plaid Cymru is ready to serve, not just those who entrusted their vote to us here but all of the citizens of Wales.”

Ap Iorwerth faced a four-hour journey from Llandudno to Cardiff to begin to make plans for his government. Reflecting on the party’s 100 years of campaigning, one party worker told the Guardian: “Politics is a game of perseverance.”

Polls throughout the election campaign had suggested Plaid Cymru and Reform UK were neck and neck in the race to be the largest party in the Senedd, pushing once dominant Welsh Labour into a distant third place.

On Friday afternoon in Llandysul in west Wales, Eluned Morgan, Wales’s Labour first minister, gave a sombre speech after losing her seat, a major indicator of the coming near-wipeout for the party.

Labour has never returned fewer than 26 seats in the 60-seat Senedd. In the new, expanded chamber, elected under a new more proportional voting system, a spokesperson said it is now expecting about 10 out of 90.

Morgan called Labour’s prospects “catastrophic” in a concession speech, in which she said she would step down as party leader, triggering a leadership contest.

“Welsh Labour has suffered a catastrophic result. Today we see the end of over a century of Labour winning in Wales. The party will need to take a look at itself, understand the depth of the challenge, and think carefully about what the public has told us … The age of two party dominance is at an end and we will need to adjust to a world where multiple parties contend for power.”

Morgan added that the vote in Wales had not been about Keir Starmer’s leadership, and she “took responsibility” for the people “rejecting Welsh Labour”.

“We owe it to the people of Wales to listen. To understand. And to rebuild,” she said.

Labour’s rout in Wales is seismic, a once in a century political and cultural shift. The party’s rebrand as “Welsh Labour” after devolution was successful on many fronts, for many years: it cemented the idea that the party was distinct and more progressive than UK Labour, putting “red Welsh water” between Cardiff Bay and Westminster. It also stopped soft nationalist voters from embracing Plaid Cymru, and ingrained devolution as the new normal.

However, support for the party has been ebbing for some years, driven by frustration at Labour’s management of public services, which have fallen behind the other UK nations on many metrics. Observers believe Starmer’s general election win in 2024 sounded the death knell, as it left the Cardiff Bay administration unable to blame a Conservative-led UK government for perceived failings.

Pollsters said Thursday’s Senedd election was very hard to predict owing to the new more proportional D’Hondt voting system, which has created 16 super-constituencies, each of which will elect six members; by the time the sixth seat on the list is decided, just a handful of votes could make the difference.

Despite using a more proportional system, messaging that the contest was a two-horse race between Plaid Cymru and Reform appears to have cut through. The last YouGov survey before Thursday’s election found “stop Reform” was the single biggest factor influencing respondents’ votes, at 14%. The second highest was immigration, at 10%.

Reform, which had hoped to become the biggest party in the Senedd – up from 1% of the vote in 2021 – appears set to come second to Plaid Cymru, but as in last year’s closely watched Caerphilly byelection, the race appears to not be as closed as predicted.

Even if Reform won the biggest share of the vote, or the most seats, it is highly unlikely to be able to form a government because most of the other parties have ruled out a coalition.

Turnout may prove to be a decisive factor in this week’s vote. For the first time, turnout in a Senedd election is expected to exceed 50%, reflecting the once-in-century changing of the guard in what was until Thursday Labour’s most unfailingly loyal heartland.