Jeremy Corbyn has accused Theresa May of being autocratic like Henry VIII by refusing to promise Parliament a vote on the final Brexit deal.
The Labour leader said it would be "extraordinary" if the Prime Minister used the royal prerogative to bypass British MPs while parliaments in other European countries got to vote on the package.
His comments come after Mrs May refused to commit herself to giving Parliament a vote when the eventual deal is thrashed out in early 2019, while questioned during a select committee hearing.
Mr Corbyn told The Guardian: "It (a final Brexit deal) would have to come to Parliament.
"She cannot hide behind Henry VIII and the divine rights of the power of kings on this one."
He added: "The idea that on something as major as this the Prime Minister would use the royal prerogative to bypass Parliament is extraordinary - I don't know where she's coming from."
The interview with the Labour leader comes as he is reportedly set to have a makeover to tap into the populist anti-establishment sentiment sweeping through politics.
Mr Corbyn defended the benefits immigration brings to the British economy and said there was a "level of exaggeration" about its negative impact.
He was also critical of calls for a work visa scheme to be introduced post-Brexit, describing the proposals as "fraught with difficulties".
And he cast doubt over the feasibility of a system in which people could only come to Britain if they had a job.
He told the newspaper: "Well then, that will work in reverse for British people going abroad, which would be difficult to implement and maybe is counter to the principles of the European market."
Companies such as Airbus, Nissan and Hitachi need to be free to move skilled workers around Europe and would leave Britain if Brexit mean higher tariffs to access the single market, he warned.
Rather than set targets to bring immigration down he instead stressed the need to tackle undercutting in the workplace.
He said: "What we need to address is exploitation, undercutting and the way in which companies are trying to destroy industrial agreements by ignoring what they should be doing, which is paying people properly and not bringing people in to undercut."
The left-winger also played down talk that Tony Blair could make a comeback to the parliamentary party.
Mr Corbyn said: "I think the nuanced differences of opinion between me and Tony Blair are quite well known ... I don't see Tony Blair and I working together. I don't think he does either."
And he dismissed reports he told friends he is ready to step down in 2019 because of his age.
He said: "Friends is obviously a very loose term these days - I've never said that. I'm very happy doing my campaigning. This is the age of the 60s - look at Trump, Clinton, Sanders, Angela Merkel - look around you.
"Sixties is the new 40s, I keep fit."