Reminders of the Labour movement’s roots are inescapable in Tredegar, south Wales: murals pay tribute to party giants Aneurin Bevan and Neil Kinnock, both of whom were born here.

The Workmen’s Hall Library is long gone, replaced by a car park, but the Cambrian Inn, which hosted early trade unions and Chartist groups, survives. The Tredegar Medical Aid Society, which Bevan used as a model for the NHS, was across the road; today it is a heritage centre paying tribute to the public health pioneers and the area’s coal-mining and steel-making past.

For decades, Blaenau Gwent was a steadfast Labour seat, at times holding the single safest majority in the UK. But in last week’s Senedd election, the Labour party lost its grip on Wales for the first time in more than 100 years. Tredegar’s post-industrial valleys constituency of Blaenau Gwent Caerffili Rhymni didn’t elect a single Labour member of the Senedd (MS): three of the six seats available under the new more proportional voting system went to pro-independence Plaid Cymru, and the other three to Reform UK.

“It’s unbelievable, really,” said Woody Woods, 61, who runs a charity supporting armed forces veterans on the floor above the heritage centre. Since the pandemic, the space has also hosted a food bank.

“It was always the way to vote Labour here, I don’t think people ever thought about not doing it … I think the party just doesn’t represent working people any more.”

Tredegar-born Alun Davies, who served as a Labour MS from 2007 until losing his seat last week, said he thought there must have been a mistake when he watched the ballots being verified on election night.

“I couldn’t see any Labour boxes in places we normally pick up votes. We started the campaign thinking we’d get the third seat, and as the weeks went by, we revised it down to four, to five … but right up until the returning officer gave us the results, I thought we still had enough support for one seat,” he said.

Polls had been pointing to Labour finishing third behind Plaid Cymru and Reform for some time, but several party insiders said they had not been prepared for such a total collapse – which also bodes ill for the next general election.

“There is no chance of Labour winning first-past-the-post anywhere in the south Wales valleys at the moment, and we’re not going to come second in many places,” Davies added.

Welsh Labour won just nine seats in the newly expanded 96-seat Senedd: previously, the party never held fewer than 26 seats out of 60. For the first time, the nationalist Plaid Cymru is at the helm with 43 seats and a secure minority government, and Reform UK, which only received 1% of the vote in 2021, is now the Senedd’s official opposition, with 34 seats.

Disbelief led Labour to ask for a recount for the sixth and final seat in Blaenau Gwent Caerffili Rhymni. While the number of Labour votes went up, so did Reform’s, and the gap widened. Davies conceded.

The seat he had hoped would preserve a Labour presence in the constituency went to Reform’s Joshua Kim, who told the BBC he was “shellshocked” and “did not think for one minute” he would be elected.

Kim, a teacher, said he wasn’t at the count because he didn’t want to miss a day’s pay doing a supply work shift. He showed up 45 minutes after the result, as the sports hall was being cleared to make way for a football match.

Many people the Guardian met in Tredegar this week said they were not interested in politics, and didn’t vote. Turnout in the constituency was 47.7%, even though this year’s Senedd election was the highest turnout on record, averaging 51.7%.

A shopper who gave her name as Helen, 63, said she voted Plaid Cymru rather than Labour for the first time because she was worried about the prospect of Reform UK running the Welsh government.

“I didn’t think Labour was the safe bet to stop Reform, so I voted Plaid Cymru. The [Reform] social media stuff you see is so divisive and full of hatred. We have to hope that Plaid can do what they say they will, but there’s no way they will be worse than the alternative,” she said.

Niamh Salkend, a new Plaid MS in the area, was elected alongside Delyth Jewell, formerly the party’s deputy leader, and Lindsay Whittle, the longtime community campaigner who held off Reform in last autumn’s closely watched Caerphilly byelection. Plaid Cymru “has a lot to prove to people”, she said.

“I was surprised Labour didn’t get that last seat, in the end it was very tight. But you can never take any vote for granted. A lot of people said they hadn’t spoken to a Labour politician in years … I think [Labour] thought the support would always be there,” she said.

“I think it sent a clear signal to the Labour party that things desperately need to change. People have put their faith and trust in us to do that.”