REUTERS George Galloway has been unveiled as a supporter of Brexit

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The former Labour MP appeared as a surprise guest at a rally of the Grassroots Out movement in central London. As the Prime Minster battled to win his EU renegotiation deal in Brussels, an estimated 1,500 people crammed into a Westminster conference centre in support of the Brexit campaign.

Speaking at the end of the night, Mr Galloway was introduced by Ukip leader Nigel Farage to a mixed reaction from those in the audience, with some walking out amid a few heckles. The Scottish politician, who was expelled from the Labour Party in 2003, urged the audience to take the opportunity to cut Britain's ties with Brussels otherwise "your children and your grandchildren will regret that you did not take it".

REUTERS The controversial politician spoke at a Grassroots Out event in London

REUTERS There was some discord when Mr Galloway was announced

He added that he had signed up to Grassroots Out to "nail the lie" that to support quitting the EU "is to be on the Right" of British politics. Mr Galloway said he was appearing "in memory of [late Labour left-winger] Tony Benn". He railed against the effect of a pending trade agreement between the EU and the US on the NHS, as well as having Britain's foreign policy decided by other EU member states through Brussels' "rich man's club".

REUTERS Galloway previously appeared on Celebrity Big Brother

EU referendum Sat, February 20, 2016 With an in/out referendum on EU membership set for June 23 this year, we're taking an overview look of David Cameron's two-day visit at EU summit at EU headquarters in Brussels and his return to Downing Street to meet with his cabinet. Play slideshow AFP/Getty Images 1 of 21 British Prime Minister David Cameron speaks at Downing Street

REUTERS The meeting came as David Cameron announced his EU deal, branded 'pathetic' by Nigel Farage

He said he wanted to prove the Brexit campaign did not have to "wallow in nationalism" adding: "I can hardly be called a Little Englander!" Tory MP Peter Bone hailed Mr Galloway's "great speech". The packed-out rally saw many locked out of the venue before the event began, with no room left inside the auditorium.