British lawmakers gathered Monday to honour a murdered colleague, attempting a rare show of unity in a heated, neck-and-neck race for votes three days before a referendum on whether to quit the EU.

As the battle raged over Thursday's vote that could change the course of post-war Europe, latest polls showed the "Remain" camp gaining ground.

Financial markets rallied as Brexit concerns eased, at least temporarily. The pound surged and Europe's major stock markets in London, Frankfurt and Paris gained more than three percent in morning trade.

"A higher chance of the UK voting to stay is a relief for markets," said Mike van Dulken, head of research at Accendo Markets.

Politicians from both sides of the fight returned to parliament, which had been in recess, to pay tribute to Jo Cox, a campaigner in favour of EU membership who was shot and killed last week in a village street in northern England.

Her alleged killer, 52-year-old Thomas Mair, was due in court on Monday after being charged at the weekend with murder.

The image of unity did not last long.

Top pro-Brexit campaigner Nigel Farage, the leader of the UK Independence Party (UKIP), accused his rivals of unashamedly using Cox's death to boost their cause.

"The 'Remain' camp are using these awful circumstances to try to say that the motives of one deranged, dangerous individual were similar of half the country or perhaps more who believe we should leave the EU," he told the BBC.

If Britain leaves the EU, it would be the first nation to do so in the bloc's 60-year history.

The British referendum has opened the prospect of other nations demanding a vote, too, perhaps placing in peril the very survival of the European project, which was born out of a determination to forge lasting peace after two world wars.

While the "Remain" camp has tried to focus on the potential economic damage that Brexit could inflict, the "Leave" campaign held out the promise of Britain taking better control of mass immigration if it leaves the EU.

Farage, however, has had to fend off attacks over his release last week of a campaign poster, which showed scores of refugees trudging through fields towards the camera with a bold, red headline "Breaking Point".

A senior member of Prime Minister David Cameron's ruling Conservatives, former party chair Sayeeda Warsi, announced she was withdrawing support for "Leave" because of the poster.

"That 'breaking point' poster really was -- for me -- the breaking point to say, 'I can't go on supporting this'," Warsi told The Times.

"Are we prepared to tell lies, to spread hate and xenophobia just to win a campaign?"

Farage dismissed her defection, describing it as a "Number 10 put-up job" -- a reference to Cameron's Downing Street office -- and casting doubt on whether she had ever really been part of the Brexit camp.

The "Leave" and "Remain" sides have now battled each other to a stalemate with each on exactly 50 percent support, according to an average of the last six polls calculated by research site What UK Thinks.

Both sides agree the stakes are high.

"I hope you will vote 'Leave', and take back control of this great country's destiny," pro-Brexit campaigner Boris Johnson -- a former London mayor who is touted as a future prime minister -- wrote in the Daily Telegraph.

"This chance will not come again in our lifetimes, and I pray we do not miss it."

Foreign Secretary Philip Hammond, arriving for a meeting with EU counterparts in Luxembourg, warned a British vote to leave the European Union would be "irreversible".

"There will be no going back," he said.

The prime minister on Sunday called on voters to pick "Remain" in a sometimes heated BBC television appearance in which an audience member accused him of appeasing an EU "dictatorship".

"If we do leave we are walking out the door, we are quitting," Cameron urged.

"I don't think Britain at the end is a quitter. I think we stay and fight. That is what we should do."

Footballing powerhouse the Premier League also weighed into the debate Monday, saying Brexit would go against the "openness" of the league and all of its clubs.

Hungarian Prime Minister Viktor Orban put aside his own euroscepticism to issue a plea to British voters.

"The decision is yours but I would like you to know that Hungary is proud to stand with you as a member of the EU," Orban wrote in a full-page advert taken out in the Daily Mail.