By Kylie MacLellan and Jemima Kelly

LONDON (Reuters) - The campaign to keep Britain in the European Union appeared to regain the upper hand on Monday, putting the pound on track for its biggest one-day gain in more than seven years and buoying the UK stock market.

Campaigning for the June 23 referendum resumed on Sunday after a three-day suspension following the killing of lawmaker Jo Cox, and three opinion polls at the weekend showed the "Remain" camp gaining momentum.

Sterling rose by as much as 2.4 percent against the dollar on Monday, heading for its biggest one-day rise since December 2008, while Britain's FTSE-100 shares index jumped 3 percent - its biggest one-day gain since mid-February. [.L] [GBP/]

The killing of Cox, a 41-year-old mother of two young children, has shocked Britain and could yet prove a defining moment in a vote that will shape the nation's role in world trade and also determine the future of the bloc.

The lawmaker, an ardent supporter of EU membership, was killed in the street by a man heard shouting: "Britain first. Keep Britain independent. Britain always comes first."

Opinion polls last week had suggested the "Out" campaign had taken the lead in a debate that has polarized Britain. But the polling at the weekend - some carried out after the murder - suggested the tide was turning the other way.

The probability of a British vote to remain in the European Union, implied by Betfair betting odds, rose to 74.6 percent on Monday, up from 60-67 percent on Friday.

Prime Minister David Cameron, who is leading the "In" campaign, has focused on the economic argument, telling voters that a Brexit would hurt wages and jobs and lead to a decade of uncertainty.

His cause was bolstered on Monday by senior executives of several carmakers, including Jaguar Land Rover, Toyota, BMW and Vauxhall, releasing statements urging Britons to stay in the EU.

"We firmly believe Britain would be better off if it remained an active and influential member of the EU, shaping European regulations," said BMW sales chief Ian Robertson.

The head of England's top soccer division - the Premier League - also weighed in, saying that its 20 clubs wanted to stay in the bloc.

"Are we better acting like we want to play our part in the world and be worldly citizens, or do we want to send a signal to the world that says, actually, we're kind of pulling the drawbridge up here?" Richard Scudamore told BBC radio.


The attack on Cox last Thursday left many voters and politicians wondering whether the campaign rhetoric on both sides - warnings of economic disaster versus uncontrolled immigration - had gone too far in a country considered a paragon of stability.

Sayeeda Warsi, a former co-chair of the Conservative Party, said on Monday she was switching her support to the "Remain" campaign because of the tactics used by the other side.

She pointed to a poster from one of the "Leave" campaigns, which used a photo of refugees walking through a field in Europe under the slogan "Breaking Point" - a message she said she did not want to form "the basis of the kind of Britain that I want to live in and to bring my kids up in".

"Are we prepared to tell lies, to spread hate and xenophobia just to win a campaign? For me that's a step too far," she told The Times newspaper.

The "Leave" camp's key argument has been that Britain would be unable to control immigration levels as long as it was in the EU, something that has struck a chord with many Britons who fear that public services are being overstretched.

The leader of the anti-EU UK Independence Party (UKIP), Nigel Farage, whose movement produced the poster, defended it as reflecting the truth. However the official Vote Leave organization condemned the use of the image - though was dismissive of Warsi's move, saying it did not remember her joining its campaign.

Parliament - which has been in recess ahead of the referendum - was reconvened on Monday for lawmakers to pay tribute to Cox.

Lawmakers avoided referring directly to Thursday's vote as they stood up to praise Cox as a dedicated campaigner, politician and mother. But several politicians urged both sides of the EU debate to try to avoid populism.

"Jo understood that rhetoric has consequences. When insecurity fear and anger are used to light a fuse then an explosion is inevitable," said Stephen Kinnock, a lawmaker who shared an office with Cox.

(Additional reporting by Estelle Shirbon, William James and Elizabeth Piper, Editing by Pravin Char)