Speaking at a rally in London yesterday afternoon, Stewart (78) told a crowd of more than 1,200 people that he "will not stand idly by" while Britain's "future is at stake".
The new People's Vote campaign has been launched in the United Kingdom, pushing for a separate referendum to ratify or reject the final Brexit deal.
James McGrory, executive director of Open Britain, which is supporting the People's Vote campaign, said: "Whether you think the government will negotiate a good deal or bad deal, Brexit is definitely a big deal". "That is why so many are demanding a People's Vote on the final Brexit deal".
MPs, celebrities and business leaders launched a campaign on Sunday to call for a vote on any final Brexit deal, stepping up a campaign to try to stop what they describe as Britain's damaging departure from the European Union.
However, both the Tories and Labour have ruled out the possibility of a referendum on the subject and time to change their minds is running out.
Stewart said the campaign was not a rejection of the Brexit result, arguing that some of the claims used by the Leave side - including a promise of additional £350 million a day for the NHS - were "misleading".
The actor, famous for productions including Star Trek and X-Men, even said his fictional character Charles Xavier would have supported the Remain campaign. And why? Because unity, common cause, well-being of society and debate were paramount to belief of this fictional character.
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"Our country's future is at stake and we will not stand idly by. If the general public demand a Individuals's Vote, politicians will fall in line".
The UK is due to leave the European Union in March 2019 following the referendum which saw Britain vote 51.9% to leave and 48.1% to remain.
"The idea we are going to have a second referendum to overturn Brexit is worthy of science fiction". "They would have voted Remain".
He said: "The cost of war was everywhere and many never recovered from it".
Stewart added: "As time has gone by, the information that we are receiving about the terms and conditions of that separation are quite unlike the terms and conditions that were spoken of so loosely during the 2016 campaign".
"They voted with a substantial majority to leave the EU".