Prime Minister David Cameron warned Sunday of abject humiliation for Britain if it quits the EU, as rival camps resumed a frequently acrimonious referendum battle shaken by the shock murder of lawmaker Jo Cox.
Four days ahead of a referendum that could rewrite the political and economic destiny of Europe, the two sides launched a last-straight push for public support as latest polls showed the Remain camp gaining ground.
"If you're not sure, don't take the risk of leaving. If you don't know, don't go," Cameron pleaded in The Sunday Telegraph newspaper.
There will be no turning back from the "existential choice" made on European Union membership on June 23, the British leader told voters, predicting dire consequences of going it alone.
"It would be a one-off and permanent diminution in our standing in the world; an abject and self-imposed humiliation for a proud and important country like ours," he said.
The tightly-fought campaign had paused for three days to mourn Cox, a 41-year-old mother-of-two and active Remain campaigner who was stabbed and shot in the northern English village of Birstall on Thursday.
It was the first murder of a sitting British member of parliament since 1990.
Her 52-year-old alleged killer, Thomas Mair, appeared in court on Saturday and shouted: "Death to traitors, freedom for Britain". He was remanded in custody and is due to appear in court again on Monday. A psychiatric report has been requested
Parishioners prayed for Cox at a Sunday church service nearby the murder scene.
"Her humanity was powerful and compelling and we would do well to recognise her as an amazing example: a 21st century Good Samaritan," Reverend Paul Knight told the congregation.
Leading Brexit campaigner Nigel Farage said Cox's murder had affected the Leave campaign's rise.
"We did have momentum until this terrible tragedy," he told ITV television.
The Sunday Mirror newspaper, citing its own ComRes polling, said there had been a "dramatic swing" in the mood of voters, attributing it to "The Jo effect".
The What UK Thinks website's average of the last six polls, conducted between June 10 and Saturday, put the Remain and Leave camps level at 50-50, excluding undecided voters.
The Leave camp had been a few percentage points ahead in recent polling, but fresh surveys showing a rise in support for Remain brought the average neck-and-neck.
A Survation poll conducted on Friday and Saturday put Remain at 45 percent and Leave at 42 percent -- the reverse of its findings on Thursday.
While the Remain camp has tried to focus on the potential economic damage that Brexit could inflict, the Leave campaign has held out the promise of Britain taking better control of mass immigration if it leaves the EU.
UK Independence Party leader Farage fended off criticism Sunday over his release of a campaign poster showing scores of refugees trudging through fields towards the camera with a bold, red headline "Breaking Point".
"The poster reflects the truth of what is going on," Farage told Sky News television, arguing that the row over the poster, released a few days earlier, might not have erupted if not for Cox's shooting.
The justice minister, Michael Gove, who supports Brexit, said: "When I saw that poster I shuddered. I thought it was the wrong thing to do."
Britain's finance minister George Osborne said it was a "disgusting and vile poster" with echoes of 1930s literature.
Osborne cautioned that Britain would be "a lot poorer" outside the EU, saying the economy could shrink by at least five percent to six percent.
"You can't predict the enormous uncertainty that exiting the EU means for Britain," he told ITV television.
Osborne said he hoped Cox's death would lead to "a less divisive political debate in our country" with "less baseless assertion and inflammatory rhetoric and more reasoned argument and facts".
Remain campaigners sought publicity with a "kiss chain" in which they said hundreds of Europeans and Britons puckered up around London's Parliament Square, and in Rome, Paris and Berlin.
Meanwhile, Britain's Sunday newspapers picked sides in their final editions before the vote.
The Mail on Sunday and The Observer gave their support to the Remain camp, while The Sunday Times and The Sunday Telegraph broadsheets backed quitting the EU.
The Sunday Telegraph declared that the EU "belongs to the past" while The Sunday Times said "Yes, we must be prepared for difficulties, but we should hold our nerve" in voting Leave.