Polls predict a dead heat with voting less than a week away but teams pledge sober campaign after death of Jo Cox

The tumultuous and often bitter EU referendum campaign has left the two rival camps – Remain and Leave – locked in a dramatic dead heat with just four days to go before the British people decide their European future.

The final Opinium/Observer poll before referendum day on Thursday puts both sides on 44%, with 10% saying they are undecided.

The online survey of more than 2,000 people was taken between last Tuesday and Friday, with 80% of the polling conducted before the murder of the Labour MP Jo Cox, which led to the suspension of all campaigning.

A YouGov poll for the Sunday Times today gives the Remain camp 44%, a one point lead over Leave. A third of the responses were taken before news of her death was made public.

Anthony Wells, director of YouGov’s political and social research team, said: “While there will be speculation about whether this movement is connected to the tragic death of Jo Cox, we do not think that it is... We are now in the final week of the referendum campaign and the swing back towards the status quo appears to be in full force.”

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Today, both sides will resume their battle to capture the votes of the undecided and to persuade people to switch sides, though both the Leave and Remain camps say that the manner of their campaigning will be more sober and less combative.

Some 79% of those who stated a clear preference said they were certain not to change their minds. Among Remain supporters, 77% said they would definitely not switch sides, while among Leave voters 81% said they would not waver.

The poll – which sets the stage for a tense and dramatic run to referendum day – suggests that, among the undecideds, more are inclined to vote Remain than Leave. When the 10% who have not made up their minds were asked which way they were leaning, 36% said they were inclining towards Remain and 28% towards Leave.

In an interview with the Observer, Kenneth Clarke, a former Tory chancellor and a strong supporter of remaining inside the EU, criticised both campaigns for making overblown claims, which he said had left much of the electorate “angry and confused”. Clarke said a referendum on such a complex and important matter should never have been called and that the result was more uncertain than the Scottish referendum in 2014 had ever been. The outcome was now “in the lap of the gods”.

Facebook Twitter Pinterest Nigel Farage launches Ukip’s new EU referendum poster campaign. Photograph: James Shaw/REX/Shutterstock/James Shaw/Rex/Shutterstock

Taking aim at the Remain camp, led by David Cameron, Clarke said it had “dramatised” Treasury reports which suggested Brexit would have serious negative effects on the economy.

But he was far more damning of the Leave camp, headed by his Tory colleagues Boris Johnson and Michael Gove, which he said had echoed the “bigotry and prejudice” of Nigel Farage. Its leaders, he suggested, had voiced opinions that he felt they did not truly hold, particularly on immigration.

Clarke added: “People have not been impressed by the campaigning. They do feel they don’t know enough about it. And they are slightly uncertain as to which way to vote.”

In a letter to this newspaper on Sunday, former Labour prime minister Tony Blair, and former deputy prime ministers Michael Heseltine, Conservative, and Nick Clegg, Liberal Democrat, head a list of prominent figures in public life who call on the British public to reject “division, isolationism and blame” by voting to stay in the EU. They say that Nigel Farage and other Leave campaigners have tried to divide the country and foster anti-immigrant feeling by promoting a sense of “us and them”.

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Privately, the Remain campaign believes that the death of Cox, a strong supporter of the EU who had praised the way in which immigration had enriched British society, will make it harder for their opponents to campaign as stridently as before on immigration issues.

Labour politicians will postpone the resumption of full campaigning until after MPs have returned to parliament tomorrow to pay tributes to Cox.

Scotland Yard will offer every MP a briefing this week on how to keep themselves and their staff safe in their constituencies.

It is understood that plans are being laid to establish a central body that can ensure that all local forces respond swiftly and appropriately to threats against MPs, following concerns that local forces have not always taken the dangers sufficiently seriously.

Facebook Twitter Pinterest Floral tributes to Jo Cox in Birstall. Photograph: Christopher Furlong/Getty Images

Writing in the Comment pages of today’s Observer, the shadow education secretary, Lucy Powell, pays tribute to her friend and colleague: “We must make sure that her memory is served not by more women now being put off by abuse and security concerns, but inspired by her life, her approach and her achievements. Losing Jo is such a deep loss to those who knew her and the causes she stood for. But in each and every one of us, in our every day lives, we must make sure that a little bit of Jo lives on.”