The Labour Party looks set to lose ground in regional and local elections, partial results showed today, pointing to a dip in support that could challenge the authority of the party's leader Jeremy Corbyn.

In the biggest test of the political mood since David Cameron's Conservative Party won a national election a year ago, Britons voted on Thursday to elect new devolved authorities in Scotland and Wales, more than 2,700 local officials across England and a new mayor of London.

The elections have been framed as a test of Corbyn's first eight months in charge of Labour after his surprise victory in an internal leadership contest last September shifted the party's political stance sharply to the left.

With more than half of the results from local government elections in England counted, Labour had lost control of a relatively modest 28 seats. But Labour's share of the vote was down nine per cent in Scotland and nearly eight per cent in Wales.

"We should have been winning by a landslide across the country with the way this Tory government's been acting and the way they've deal with the country," Labour MP John Mann told the BBC.

"We're holding our own, but we should be doing dramatically better than this."

The lone bright spot for a party struggling to hold the government to account may be London's mayoral election, where opinion polls show Labour candidate Sadiq Khan is expected to become the first Muslim to take the post later on Friday.

If overall losses are confirmed, they will represent a negative verdict on Corbyn's leadership, which has so far been dogged by vicious disagreements with a moderate core of Labour MPs and a string of embarrassing crises – most recently over anti-Semitic views expressed by party members.

The scale of Labour's problems were most evident in Scotland, where the Scottish National Party (SNP) was, as expected, on course for a second consecutive majority.

But Labour, who dominated politics in Scotland for decades, could find themselves beaten into third place by the Conservatives.

"What we're seeing tonight is the SNP replacing Labour. The collapse in Labour support is quite staggering," SNP leader Nicola Sturgeon told broadcasters.