New opinion polls are now suggesting Remain has established a consistent lead
A personal attack on David Cameron has been launched by one of his closest political allies, Michael Gove, and the Prime Minister's rival for the Conservative crown, Boris Johnson.
In a direct challenge to Mr Cameron's case that Britain will be "stronger, safer and better off" in the European Union, the top Tory pair claim staying in would make it hard to identify terrorists and keep the UK safe.
And in a highly controversial move, Mr Gove is also blaming the EU for the worst upsurge in the far right "at any time since the 1930s" and accused Brussels of fuelling a rise in "Hitler worshippers".
Interviewed in The Sunday Times, the Justice Secretary added: "Golden Dawn in the Greek parliament are explicitly Hitler worshippers and that is a direct consequence of what has happened with Greece and the single currency."
The Mayor of London told the newspaper that the EU has taken decisions which damage Britain's ability to mount serious surveillance operations against terrorists.
But speaking on The Andrew Marr Show, Mr Johnson appeared to concede that an initial period of dislocation, uncertainty and job losses may follow a vote to leave the EU. "It might or it might not," he said.
German Finance Minister Wolfgang Schaeuble also expressed concern about what would follow if Mr Johnson got his wish of a UK exit.
"For years we would have such insecurity that would be a poison to the economy in the UK, the European continent and for the global economy as well," he told Marr.
Meanwhile, Mr Johnson has described a decision by the British Chambers of Commerce to temporarily suspend its director general John Longworth as "absolutely scandalous", after he backed leaving the EU in a Sky News interview on Thursday.
Responding to a claim in The Sunday Telegraph that 10 Downing Street put pressure on the BCC to suspend him, Mr Johnson said in a statement: "It cannot be right that when someone has the guts to dissent from the establishment line, he or she is immediately crushed by the agents of Project Fear."
A No 10 source has denied any Downing Street involvement, telling Sky News: "This is simply not true. This is a matter for the BCC. No pressure was put on the BCC to suspend John Longworth."
The latest "blue on blue" assault on Mr Cameron comes in a weekend when activists campaigning for Britain to leave the EU hit the high streets in towns like Wokingham, the constituency of Eurosceptic Tory MP John Redwood, in their biggest day of action yet.
Another Tory Eurosceptic MP, Peter Bone, was campaigning in Croydon. In all, it's claimed 3,500 Leave activists staffed more than 300 street stalls nationwide and handed out leaflets.
But pro-EU MPs were out campaigning, too. In west London, Labour's Andy Slaughter took the Remain campaign to shoppers in Hammersmith.
New opinion polls are now suggesting Remain has established a consistent lead.
Polls conducted by YouGov on 28 January and 3 February both put Leave ahead by margins of up to nine points.
But new surveys on 1, 2 and 3 March put Remain in front by two, five and three points respectively.
The polls also suggest almost one in five voters are undecided, and the security warnings by Mr Gove and Mr Johnson will lay them open to the charge that they are using the same scare tactics that Outers claim are being employed by the Government.