Labour and the Conservatives face a historic humiliation in a key election next year, new polling suggests. The two parties are behind Welsh nationalists Plaid Cymru and Reform UK in the race to become the biggest party in the Welsh Parliament, the Senedd.

Plaid Cymru is the choice of 30% of voters with 25% saying they would vote for Nigel Farage’s Reform UK. The findings signal that Reform UK is positioned to become the “predominant right-of-centre party” in Wales.

Labour has the support of just 18% with the Conservatives on 13%, according to the Barn Cymru poll carried out by YouGov for ITV Cymru Wales and Cardiff University. The Liberal Democrats are on 7% with the Greens on 5%.

Labour faces the worst result since devolved government began in Wales in 1999. Wales has never had a non-Labour First Minister but the polls suggest Plaid Cymru leader Rhun ap Iorwerth could be on the verge of making history.

A devastating result for Labour and the Conservatives in Wales would heighten fears in Westminster that the parties are at risk from Reform UK in the general election. Last week’s local elections in England saw the Conservatives lose 674 seats and Labour shed 187 while Reform won UK won 677 and the Lib Dems increased their tally by 163.

When it comes to Westminster voting intention in Wales, Plaid Cymru is tied with Reform UK on 24%, ahead of Labour (20%), the Conservatives (13%), the Lib Dems (9%) and the Greens (7%).

The Tories won 14 Welsh MPs in Boris Johnson’s 2019 landslide but failed to win a single constituency in the July general election.

In an analysis of the findings by Cardiff University’s Jac Larner, quoted by ITV Wales: “Reform UK will undoubtedly be encouraged by achieving their strongest ever performance in a Welsh opinion poll (exceeding even the Brexit Party’s previous high watermark of 23% in 2019). These figures suggest Reform UK is positioned to establish itself as the predominant right-of-centre party appealing to the British-identifying bloc of the Welsh electorate.”

The polling will make difficult reading for Sir Keir Starmer. When asked to rate his performance, 62% said he was doing very or fairly badly. Only 27% said he was doing very or fairly well.

Welsh Labour First Minister Eluned Morgan has said she is willing to “call out” Westminster Labour when it “gets it wrong for Wales”. Ms Morgan has pledged not to “stay silent” if the UK Government takes decisions she thinks “will harm Welsh communities”.

She has pressed for a rethink of the winter fuel cuts, stating that it is the “people of Wales before party for me”.