David Cameron has warned EU "out" campaigners that Brexit will not stem the flow of migrants coming into the UK.
Hours after firing the starting gun in the referendum campaign, the Prime Minister sought to take head on one of the central claims of the leave camp.
In an interview with The Sunday Times, he said the EU would insist upon continued free movement of labour - as well as a contribution to Brussels' coffers - as the price of a free trade deal if the UK left.
"So far, the EU has never given full access to the single market without insisting on a contribution to the budget and free movement," he said.
The Prime Minister was given an early boost with a poll by Survation for The Mail on Sunday putting support for remaining in the EU on 48% with 33% in favour of leaving and 19% undecided.
It came as Boris Johnson was expected finally to declare which side he will be supporting, ending months of prevarication.
"Outers" have been hoping the London mayor could be the figurehead for their campaign has been looking for, with the ability to connect with voters in a way few other Westminster politicians can match.
Speculation that he is leaning towards the leave camp was heightened with the disclosure that he had a private dinner last week with Michael Gove, who declared on Saturday that he would be joining the "out" campaign.
The Mail on Sunday published pictures of the Justice Secretary and his wife, the journalist Sarah Vine, arriving at Mr Johnson's home in Islington, north London, on Thursday.
Mr Cameron will continue to make his case for voters to back his re-negotiation deal - finalised in marathon summit talks in Brussels - in a high-profile television interview with BBC1's The Andrew Marr show.
However his words on migration immediately put him at odds with Chris Grayling - one of six ministers to announce he would be campaigning for "out" following Saturday's historic Cabinet meeting.
Despite the "emergency brake" on migrant benefits secured by Mr Cameron in his re-negotiation deal with Brussels, the Leader of the Commons said he saw no immediate end to the pressures on the UK.
He said only by leaving the EU could Britain put in place the measures it needed to control the numbers entering the country.
"The national living wage will have a boosting effect on the attraction of Britain as a place to come and work and I don't see any obvious sign of the migration pressure on the UK ending any time soon," he told The Sunday Times.
"I think we do need to take steps to place some limits on those pressures for the future. We clearly can't do that within the European Union."
Although Mr Cameron has given ministers the freedom to support either side in the referendum, the exchange underlined the difficulty he faces in holding together the Government during a bitterly contested four month campaign.
Business Secretary Sajid Javid - who has declared for the "in" camp, despite strong Eurosceptic leanings - underlined just how conflicted many Conservatives felt.
"My heart says we are better off out. My head says it's too risky right now," he wrote in an article for The Mail on Sunday.
"I'm no Europhile, but nor am I prepared to risk undoing all that work and casting aside all the sacrifices we asked of this country while the post- Brexit talks drag on and investor confidence wavers."
:: Survation interviewed 1,004 people by telephone on Saturday.