Scottish Labour leader Kezia Dugdale has thrown her support behind Owen Smith just a day after Mayor of London Sadiq Khan also backed the Pontypridd MP in his campaign to dethrone Jeremy Corbyn. The endorsement comes as ballots are sent to hundreds of thousands people eligible to vote in the Labour leadership election, with the result being announced on 24 September in Liverpool.
Dugdale warned that Corbyn spoke "only to the converted" and claimed the left-winger would fail to unite Labour.
"We can't pin our hopes on a leadership who speak only to the converted, rather than speaking to the country as a whole," she wrote in The Daily Record.
"My only public comment on Jeremy's leadership before this contest was to say he had lost the confidence of his parliamentary colleagues. That's a fact."
Dugdale added: "I don't think Jeremy can unite our party and lead us into government. He cannot appeal to a broad enough section of voters to win an election.
"I believe Owen can.
"There will be some who think I should stay out of this contest, but now, more than ever, our party needs leaders who will stand up and be counted.
"As the most senior female elected leader in the Labour Party across the UK, I feel a particular responsibility to speak out.
"I believe Owen can deliver the Labour government this country needs, so he gets my vote."
Scottish Labour have faced difficulty since the general election, which saw the party reduced to just one MP north of the border. The 2016 Holyrood elections also dealt a blow, with the Scottish Conservatives overtaking Scottish Labour to become the second largest party in the Scottish Parliament.
Elsewhere, Khan also questioned Labour's electbility under Corbyn, with the most recent opinion poll from YouGov for The Times, of more 1,600 people between 16 and 17 August, giving the Conservatives an eight point lead over Labour (38% versus 30%).
Corbyn, who remains favourite to retain the Labour leadership after almost attracting 60% of the vote in 2015, announced a raft of policies on 21 August to "extend democracy" across the UK and inside Labour .
Corbyn reaffirmed his commitment to axe the unelected House of Lords and, among other things, to introduce new collective and individual rights at work.
"I am determined to democratise our country from the ground up, and give people a real say in their communities and workplaces," he declared.
"We need to break open the closed circle of Westminster and Whitehall, and of the boardrooms too.
"Decisions in Britain are overwhelmingly taken from the top down. And that's crucial to why our country is run in the interests of a privileged few.
"That has to change – so that the country works in the interests of the millions, and not just the millionaires.
"I believe in the wisdom of ordinary citizens. That's why we are launching proposals to extend democracy in every part of public life: in national politics, communities, the economy and the workplace – and in our own party."