German Finance Minister Wolfgang Schaeuble on Friday issued a stark warning to Britain that it would face costly barriers to the European Union trade zone if it left the bloc.
The comments by one of Europe's most senior officials is a blow to Brexit supporters, who have argued that Britain could negotiate deals to access the single market similar to those in place for non-members Norway and Switzerland.
"That won't work," the veteran minister told Germany's Der Spiegel weekly, which on Saturday plans to publish a German-English edition at home and in Britain with "Please don't go!" on the cover.
"That would require the country to follow the rules of a club which right now it wants to leave."
European Commission president Jean-Claude Juncker last month said that British "deserters will not be welcomed with open arms" by European partners if Britain votes to leave in the June 23 referendum, but Schaeuble's intervention is the most explicit threat so far.
Vote Leave chief executive Matthew Elliott hit back, saying there was "no question about it, Britain will still have access to the single market.
"It would be perverse of the eurozone to try to create artificial barriers -- and would do far more damage to them than to anyone else," he said.
The EU accounts for 47 percent of British exports and 54 percent of imports, according to latest government figures.
- Labour voters left confused -
With opinion polls on a knife-edge, senior figures in Britain's opposition Labour party made an impassioned plea to stay in the EU amid fears their failure to get out the left-wing vote may result in a Brexit.
Labour leader Jeremy Corbyn is officially backing the "Remain" campaign but has been keeping a low profile.
Former party leader Ed Miliband acknowledged in a BBC interview: "Some Labour voters don't know where we stand at the moment."
Corbyn has refused to share a platform with Prime Minister David Cameron and there are concerns some Labour voters will abstain or back a Brexit to give the Conservative leader a bloody nose.
But Miliband accused the "Leave" campaign, also backed by the anti-immigration UK Independence Party (UKIP) leader Nigel Farage, of "trying to perpetrate a fraud on Labour voters".
"They want to get out of the European Union not to improve workers' rights but to sweep them away," said Miliband, who stepped down after losing last year's general election.
- 'People's revolution' -
The economic turbulence of a Brexit would create "a massive black hole in the public finances, and an unfair Tory government that will make ordinary families pay for it through further cuts and tax rises", Labour deputy party leader Tom Watson told the Daily Mirror tabloid.
A YouGov poll for The Times this week found public opinion evenly split between leaving or staying in the EU, but Labour voters favoured "Remain" by 61 percent to 26 percent, with the remainder either not voting or undecided.
However, Labour voters were also marginally less likely to say they would definitely vote.
The leaders of 10 major trade unions came out in favour of staying in the EU this week, but many workers blame the mass migration caused by the EU's freedom of movement rules for driving down wages.
Two backbench lawmakers, John Mann and Dennis Skinner, on Friday announced that they were backing Brexit as the best way to secure workers' rights.
In an article for The Sun tabloid, Mann said the arrival of hundreds of thousands of EU migrants into Britain each year was "worsening inequality".
He added: "A people's revolution is under way. This is about returning power to the people."