By Andrew MacAskill and William Schomberg
LONDON, June 8 (Reuters) - An anti-Brexit group backed by billionaire financier George Soros launched a campaign on Friday targeting lawmakers, in an attempt to strengthen its call for a second referendum that could stop Britain's exit from the European Union.
With under 10 months left until Britain is due to leave the EU, opponents of Brexit are exploring ways to thwart what they call Britain's biggest mistake since World War Two.
The group, "Best for Britain," unveiled a timetable for another referendum on membership of the EU once Prime Minister Theresa May has obtained a final exit deal in the autumn.
It wants the new referendum on the exit terms early next year.
Best for Britain argues that both sides of the Brexit debate should welcome a second poll in less than three years to finally settle the question of Britain's membership of the EU and said that the issue was diverting time and energy from other political concerns.
"Give us a straight choice between your deal on the best terms you can get or decide to stay in now we see the costs of leaving," said Mark Malloch Brown, the group's chairman, at an event in London.
"The people instructed the government to negotiate a withdrawal from the EU, now the government owes people an answer: what terms have you got, are we going to be worse or better off?"
Best for Britain said a referendum could be held in early 2019 and the EU would allow Britain to have a second referendum as late as February or March.
The group intends to campaign to persuade both Conservative and Labour Members of Parliament to back its demands for a second referendum.
The group will spend 500,000 pounds ($670,000) donated by Soros, on billboards, newspaper advertising and social media to encourage people to change their minds.
Soros has angered some Brexit supporters who say he is seeking to undermine the outcome of the 2016 referendum which resulted in a 52-48 percent result in favour of leaving the EU.
Malloch Brown defended taking money from the Hungarian financier, saying he has a home in London, he went to university there and his business has an office in the city.
The prime minister and the opposition Labour Party have repeatedly ruled out the possibility of a second referendum, saying Britain will leave the EU in March next year.