Speaking at a rally in Camden, north London, Sir Patrick told a crowd of more than 1,200 people that he "will not stand idly by" whilst Britain's 'future is at stake'.
Actor Patrick Stewart, a supporter of the People's Vote campaign, said campaigners were not asking for a second referendum, but rather wanted "another chance to consider what the terms of this divorce are going to be".
Opening the London launch of People's Vote - a new grassroots movement campaigning for a referendum on the final deal - the 77-year-old British actor said Brexit will mean he will show his new Blue passport 'with less pride'.
Both Prime Minister Theresa May's Conservatives and the main opposition Labour Party have ruled out a second referendum after Britons voted 52-48 percent to leave the bloc nearly two years ago, and the country is due to leave the European Union in March next year.
Green Party co-leader Caroline Lucas, also at the event, reassured attendees, saying: "We'll do everything we can in Parliament for a people's vote". What the Government comes back with, not what was promised in the referendum, will be the real deal.
Stewart said his alter egos, Star Trek's Jean-Luc Picard and the X-Men's Charles Xavier, were "excellent, admirable" individuals concerned about the well-being of all and "they would have voted Remain".
He added that "history and emotion" led him to want to stay in the EU.
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"They voted with a substantial majority to leave the EU".
"I'm a war baby and growing up a lot of the world was not good".
The transition deal will not be finalised unless Britain and the other 27 European Union countries agree on divorce terms by October - Brussels' deadline to wrap up talks.
The People's Vote campaign would argue the public, not MPs, should get the final say on Brexit, he said.
Asked what he would say to people who voted for Brexit, he told the programme "what we are doing is in their benefit".
"I think we'll get a great result and we'll be able to have, not only a very big free trade deal with our friends and partners across the channel, but we'll be able to boldly go to areas we perhaps neglected over the past five years".