Britain's opposition Labour Party on Monday scrambled to make the case for the EU as stock markets slipped and a new poll showed Brexit backers in the lead ahead of next week's knife-edge referendum.
With the latest poll giving the "Leave" camp a six point lead, former Labour prime minister Gordon Brown pleaded the EU's case to left-wing voters, while Irish Prime Minister Enda Kenny also began a tour of the United Kingdom to campaign against Brexit.
"Britain can't build a relationship just on its own, only the EU coming together can offer the economic incentives and strategic support so we can deal with the causes of terrorism," Brown said during a speech in Leicester, central England.
The former Labour leader insisted that the only way to defeat "gangmasters" fuelling illegal immigration was "through cooperation" with the EU.
Many Labour supporters plan to vote "Leave" on June 23 due to concerns over levels of immigration from the European Union to Britain, leading to fears in the "Remain" camp that their votes could be decisive.
A Guardian/ICM poll published Monday gave "Leave" a 53 percent to 47 percent advantage, meaning that an average of the last six polls compiled by the WhatUKThinks website puts "Leave" ahead on 52 percent.
Conservative Prime Minister David Cameron appeared to have temporarily stepped back from campaigning to allow Labour to make the case to its side.
Labour leader Jeremy Corbyn has made only a limited number of appearances in the referendum campaign.
The voice of Brown, who between 2007 and 2010 served as the party's most recent prime minister, is seen as influential with Labour supporters.
His intervention late in the Scottish independence referendum campaign in 2014 was credited with helping the successful push to stay in Britain.
On the other side of the debate, a group campaigning for Britain to leave the EU came under fire after tweeting that EU membership could lead to an "Orlando-style atrocity".
Home Secretary Theresa May, who wants Britain to stay in the EU, said the post was "utterly irresponsible".
- 'Investors seek safety' -
World stock markets slid further on Monday after several recent opinion polls suggested the "Leave" camp could win.
Tokyo's main stock index dived 3.5 percent to a two-month low by the close on Monday.
Worries over Britain's EU membership vote sparked a rally in the safe haven yen currency, which in turn hammered shares in Japanese exporters.
"Risk aversion continues to drive markets ahead of the US open on Monday, as investors seek safety ahead of a number of key risks events," said Craig Erlam, senior market analyst at Oanda trading group.
Around 1500 GMT, London's FTSE 100 index was down 1.2 percent compared with Friday's close.
In the eurozone, Frankfurt's DAX 30 index and the CAC 40 in Paris were down around 1.8 percent.
The British pound hit two-month lows against both the euro and dollar.
- End of Western civilisation? -
European Council President Donald Tusk also issued a stark warning on Monday, saying that Brexit could spell the "destruction" of Western political civilisation.
Do you know why these consequences are so dangerous? Because in the long-term they are completely unpredictable," he told the German mass circulation daily Bild.
"As a historian, I am afraid this could in fact be the start of the process of the destruction of not only the EU but also of Western political civilisation," Tusk said.
Also on Monday, Ireland's Kenny began a tour of the UK to try to shore up support for "Remain", particularly among the estimated 600,000 Irish citizens living in Britain who can vote.
"It is no secret that the Irish government very much wants the UK to stay as a member of the EU," Kenny said during a speech in Belfast, Northern Ireland.
"I very firmly believe that our common membership of the EU provided an important backdrop to the Irish and UK governments working together to secure peace in Northern Ireland."
Ireland, an EU member which is also part of the eurozone, is concerned that Brexit could hit its trade ties with Britain and mean the restoration of customs checks along its border with Northern Ireland.