BRIAN Cox has launched a scathing attack on Boris Johnson, branding the Prime Minister a serial liar.

The award-winning Scottish actor says the UK has “lost the plot” as he commented on the “extraordinary” political climate.

The Succession star, who switched to back the SNP in 2015, also hit out at Labour.

Appearing on ITV’s Peston show, Cox spoke about the challenges facing young actors as he bemoaned a lack of social mobility in modern Britain.

He continued: “We’ve lost the plot on so many levels, and now we have a compulsive liar who’s actually leading the country – and that liar is tolerated.

“I find it extraordinary. I come back here and there’s this man who cannot help … he wouldn’t know the truth if it landed on him.”

WATCH: Starmer stunned as Brian Cox destroys Labour and makes case for independence

Cox also laid into Labour, the party he formerly supported, in front of Labour shadow cabinet member Emily Thornberry watched on.

The actor narrated adverts for Tony Blair’s New Labour in the 1990s, and starred in a party political broadcast for Scottish Labour but in 2012. But he switched to the SNP in 2015 and has become a passionate supporter of Scottish independence.

Cox said he thought New Labour would usher in a “magnificent” period when they swept to power in 1997.

“But then Iraq, and the million-man march went for nothing,” he added.

Tory MP Bob Seely had the task of defending Johnson on the Peston show.

The National:

He claimed the Conservative leader has “many fine qualities”, while acknowledging: “He has occasionally skirted around issues in a way that he shouldn’t.

“I hope we get over this period and I hope he makes his apologies and I hope they mean something.

“I’ve found him to be just really intellectually engaged and a good person when I deal with him, so I’m going to stick up for him in that sense. I wish this wretched partygate hadn’t happened.”

Ahead of the 2020 Holyrood election, Cox provided the voiceover for an SNP campaign video.

In it, he explained why Scotland “has what it takes to be an independent country”.

Cox also spoke about Scottish independence recently in an interview with the Sunday Times to promote his new book, Putting the Rabbit in the Hat.

“The United Kingdom is not united," the Succession star said. “We have been at the tap end of the bath long enough. We deserve to have our back against the smooth surface for a while.

“It’s been tough and it is tough, and I think Nicola [Sturgeon] does an incredible job and it’s a tough job."