LONDON (AFP) - British lawmakers prepared to pay tribute to a slain MP in a rare show of unity on Monday (June 20) as the EU referendum campaign entered its final stretch with rival camps vying to seize the momentum.

Politicians will return to parliament, which had been in recess, for a special sitting to pay tribute to Jo Cox, a campaigner in favour of EU membership and refugee rights murdered on a village street last week.

Politicians on both sides of the debate sought to lay out their case to voters with just three days of campaigning left.

"You can change the whole course of European history," pro-Brexit campaigner Boris Johnson wrote in the Daily Telegraph ahead of Thursday's ballot.

"I hope you will vote Leave, and take back control of this great country's destiny," he said.

"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.

Prime Minister David Cameron 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."

The prime minister will speak at a campaign event later on Monday, and billionaire entrepreneur Richard Branson is set to launch his own campaign pleading to remain in the EU.

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, while Hungary 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.

The Leave and Remain sides have battled each other to a stalemate with each on exactly 50 per cent support, according to an average of polls calculated by research site What UK Thinks.

The vote on Thursday could see Britain become the first country to leave the 28-member European Union, a prospect that rattled markets last week, when the Leave side appeared to be gaining.

Opposition Labour leader, Jeremy Corbyn, is due to be grilled by a live television audience later on Monday as he makes his case for Remain.

The murder of fellow Labour party politician Cox, who was known for her pro-EU stance and refugee advocacy, caused widespread shock and questions over whether the tone of the campaign had been divisive.

Her alleged killer, Thomas Mair, replied "Death to traitors, freedom for Britain" when asked to give his name at a court appearance on Saturday.

Mair is due at London's Old Bailey court later Monday for a bail hearing.

Criticism has focused on a poster unveiled by the Leave campaign showing a queue of migrants and refugees on the border of Slovenia, with the words "Breaking point" in large red letters.

Finance minister George Osborne called it "disgusting and vile" and said it had "echoes of literature used in the 1930s".

Another senior Conservative politician, Sayeeda Warsi, announced she was withdrawing support for Leave due to 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?"

The "Leave" side has in turn accused "Remain" of scaremongering with its warnings of recession if Britain leaves the EU.

Farage has dismissed criticism of the poster and denied stirring hatred, but conceded Sunday that the murder of Cox may have tempered the upward march of his campaign.

"We did have momentum until this terrible tragedy," he told ITV. "When you are taking on the establishment, you need to have momentum."