Ian Murray, who is running for the UK party deputy leadership, said Mr Leonard had “come in on the coattails of Jeremy Corbyn” but that Labour had to change because the “Corbyn experiment has not worked”.

Mr Murray unveiled a video recorded by former Prime Minister Tony Blair, who has endorsed him for the deputy leadership despite saying he would stay out of the ongoing UK leadership election to avoid “damaging” any of the candidates.

The Edinburgh South MP, who faces an uphill battle to catch two Corbyn loyalists leading the deputy leadership race, warned that Labour had to win in Scotland to have any hope of returning to government.

Scottish Labour leader Richard Leonard at a UK leadership hustings in Glasgow

“If we only return one Scottish Labour MP in the next general election in 2024, the consequence is that we need to win every single non-Scottish seat which is currently Conservative, all the way down to and including Jacob Rees Mogg’s Northeast Somerset, with a 13 percent swing,” Mr Murray said.

“If we return 16 Scottish Labour MPs, we still have to win every single seat up to and including Croydon South, on an 11 percent swing. The 1997 landslide swing was just over nine percent.

“So without winning seats in Scotland, it’s very difficult to envisage another Labour Prime Minister walking through the door of Number 10. Which is why having a Scot at the top of the Labour Party is so important.”

Mr Murray has billed himself as the ‘change candidate’ and says his opponents - shadow Education Secretary and frontrunner Angela Rayner, shadow Justice Secretary Richard Burgon, shadow Women and Equalities Secretary Dawn Butler, and shadow Sport Minister Rosena Allin-Khan - all represent continuity with the Corbyn period.

Asked about the position of Richard Leonard, another close ally of Mr Corbyn’s, the MP said: “Richard's now got an opportunity to wipe the slate clean, hasn't he?

“Richard did come in on the coattails of Jeremy Corbyn. It hasn't worked out. The election results across the UK, but also in Scotland, show that it hasn't worked out.

“The biggest impediment to the Labour vote in the 12 December elections was the leadership of the Labour Party, at UK level. That was my experience and that's been reflecting on what the public told us.”

Mr Murray said it was “terribly important” that the Scottish Labour deputy leadership contest, which is taking place at the same time, is won by veteran MSP and Corbyn critic Jackie Baillie.

“It's important now that Richard builds a team around him that can build that broad coalition of support, and it's also important that we get it right in two ways,” Mr Murray said.

“One, on the constitutional arguments - no to a second independence referendum and no to independence, because there's no mandate for the former and principally against the latter so let's not facilitate the means if we disagree with the ends. Let's get that starting point in place and campaign heavily on that.

“Let's secondly make sure we're making the arguments about why independence will be bad for Scotland - and all those arguments are tenfold now that Brexit gives us the examples of borders and of turning away from your closest trading partners - and let's put together a policy platform that talks about the future. What does Scotland look like in ten, 20 or 30 years time?

“The best thing we can do as a UK Labour Party to help the Scottish elections in 2021, is to show that the Labour Party can be a credible county government.”

Asked if his and Ms Ballie’s candidacies represented the threat of a “coup” against Mr Leonard, Mr Murray replied: “No, we're using these deputy leaderships as a staging ground to try to try and get the Labour party back into government. That's where we need to be… the Corbyn experiment has not worked.”

But in a warning to Mr Leonard if the Scottish Labour leader fails to change course, Mr Murray added: “I will say to Richard Leonard what I said to Jeremy Corbyn back in 2017: there's a route to power. That route to power has to be listening to the public, and if they don't listen to the public, then the ballot boxes will reflect that.”

Ian Murray warns Richard Leonard to 'wipe slate clean' and reject independence