Starmer's Labour party suffers landmark defeat in Wales

 Britain's PM Keir Starmer speaking during the weekly session of Prime Minister's Questions (PMQs) in the House of Commons in London. [AFP]

Britain's ruling Labour party lost a regional parliament seat in its traditional stronghold of Wales Friday, an ominous sign for embattled Prime Minister Keir Starmer ahead of wider local elections next year.

Labour, which has been the largest party in the Welsh Parliament since its creation in 1999, came a dismal third in the by-election in Caerphilly, south Wales, where it had never lost before.

Welsh nationalist party Plaid Cymru took the assembly seat, with Nigel Farage's anti-immigration Reform UK second, surging to 36 percent of the vote and helping to restrict Labour to just 11 percent.

Victorious Plaid Cymru candidate Lindsay Whittle said at the count that Labour's vote "has just vanished into thin air".

Labour has for more than a century dominated politics in traditionally working-class south Wales, where once dominant heavy industry like mining has largely shuttered in recent decades.

The party has held the Caerphilly seat in the UK parliament since its 1918 creation, and in the Welsh parliament, known as the Senedd, since it was formed almost 30 years ago.

Labour, whose nationwide poll ratings have plummeted since Starmer took power in July last year, face a daunting task holding on to voters next May at local elections across Wales, London and Scotland, where the pro-independence Scottish National Party are tipped to remain in power.

Reform UK has been leading by double-digit margins in many national polls for much of this year, although the next general election is not expected until 2029.