Sir Patrick claimed the characters in his most famous screen roles, Star Trek's Captain Jean-Luc Picard and Charles Xavier from the X-Men, would have supported Britain's European Union membership.
Sir Patrick, 77, was joined by MPs from across the political spectrum as he opened a People's Vote rally attended by more than 1,200 people in London.
A new campaign group, backed by a star from the Star Trek movies, was launched Sunday calling for a public vote on a final Brexit deal.
"Since the 2016 referendum, there has been a concerted effort by many - including government - to shut down debate about Brexit and to tell us that we should simply trust ministers to get on with it because Brexit is an irreversible process".
Both the Conservatives and Labour have ruled out a second referendum.
And they faced jibes from Brexit supporters after recruiting Star Trek actor Sir Patrick Stewart to be a leading spokesman for the group. 'Our country's future is at stake and we will not stand idly by, ' he said.
The organisation wants the public, not politicians, to have the power to approve or reject the final Brexit deal and held a rally in London yesterday.
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It is hoped that Britain and the European Union can reach an agreement on a Brexit deal by October in order for this to be ratified by both parliaments before March next year, when the UK's membership of the bloc formally comes to an end. We are short on time, but we have people power on our side. Tory Remainer Anna Soubry told the crowd in London that, whatever deal is done, "you and your grandchildren will be less prosperous than you are now".
Stewart, who portrayed psychic Professor Charles Xavier in the Hollywood series after finding worldwide fame as the Star Fleet captain, appeared on the BBC's Andrew Marr Show yesterday as he prepared to launch the People's Vote campaign group.
Appearing alongside Mr Umunna on ITV's Peston on Sunday, she said: "A second referendum, Chukka, which is what you're really campaigning for, is never going to happen".
Stewart told the BBC he is supporting the People's Vote because the terms and conditions coming out of the negotiations are very different from those discussed during the 2016 referendum campaign.
"I'm a war baby and growing up a lot of the world was not good".
"They voted with a substantial majority to leave the EU. We're now trying to deliver on that mandate from the people".
He said Brexit would allow Britain to boldly go to areas it has neglected in the search for new free trade deals.