Brexit's biggest hitters have gone into battle to seize back the referendum initiative from US president Barack Obama by putting immigration at the top of the agenda.
Justice Secretary Michael Gove has warned the UK faces a migration "free for all" unless it breaks away from Brussels as the Leave camp moved to exploit an admission from the Government that EU free movement of labour rules make it harder to curb immigration.
The intervention came as London mayor Boris Johnson came out fighting after being roundly condemned over his highly personalised attacks on Mr Obama.
And former Cabinet minister Iain Duncan Smith said Mr Obama was wrong to suggest that the UK would be at the "back of the queue" for a trade deal, claiming there were many politicians in Washington eager for an agreement with post-Brexit Britain.
The London mayor turned his fire on Prime Minister David Cameron with a scathing assault accusing him of achieving "two-thirds of diddly squat" in negotiations with Brussels for a special deal for Britain on immigration and other key demands.
In his Daily Telegraph column, Mr Johnson warned the Remain side not to "crow too soon" that the Leave side had been "bombed into submission".
"The Prime Minister asked the EU for reform and got two-thirds of diddly squat. That deal shows how contemptuously we will be treated if we remain," Mr Johnson said.
Former work and pensions secretary Mr Duncan Smith backed Mr Johnson over his attack on "part-Kenyan" Mr Obama, although acknowledged it may have been "clumsy".
"He simply referred to some of the reasons why he may have a particular lack of regard for the UK," he said.
"He was trying to illustrate, I think, and you may say it was clumsy, but it was an illustration of the reality that he is the president of the United States, that this president came over to the UK and, in essence - I was quite surprised - ended up what appeared to be lecturing the UK, British citizens, as to what they should do in the forthcoming referendum."
He told BBC Radio 4's Today programme that "nobody knows" how long it takes for trade deals to be completed.
"This is not a major point, nobody knows. The president doesn't know. The president doesn't know if it takes five or 10 years," he said.
"It takes as long as both sides want to be able to make it take."
He added: "All of the politicians in Washington told me last week they wanted to do a deal with us and they don't agree with Mr Obama."
Mr Gove insisted potential new members of the EU such as Turkey and Albania posed a "direct and serious threat" to public services such as the NHS, and social harmony.
The Cabinet heavyweight wrote in The Times that the health service faced "unquantifiable strain" unless Britain quits the EU.
Home Secretary Theresa May was making her first major speech of the campaign since backing Mr Cameron's Remain stance after she admitted on the BBC's Andrew Marr Show: "Yes, free movement makes it harder to control immigration, but it does not make it impossible to control immigration."
Mrs May, who insisted that border control and immigration were two separate things, also acknowledged migrants would look at the rises in the national living wage when making a decision on whether to come to Britain.
Mr Duncan Smith said immigration was "out of control" and "poorer people" had seen their livelihoods damaged by EU citizens coming to the UK.