The Conservatives have claimed immigration would “surge” if Jeremy Corbyn became Prime Minister.
But Labour branded it the latest from the Tory Party’s “make-believe research department”.
The Tories said analysis of new research into Labour’s proposals for open borders suggests net migration “could increase to 840,000 per year if Mr Corbyn is elected as Prime Minister”.
They said the analysis, described as “fake news” by Labour, was based on official figures and the Government’s own methodology.
According to the party, the research shows extending free movement to the rest of the world would result in average net immigration to the UK of 840,000 a year over the next 10 years.
“This is equivalent to the combined populations of Manchester and Newcastle moving to the UK every single year,” the Tories said.
“This means that levels of net migration would more than treble if Labour introduced their proposals for completely open borders.”
The Tories added the analysis is “deliberately cautious and is likely to provide a significant underestimate of net inflows from non-EEA countries under Labour’s plans”.
The party also said maintaining free movement with existing EEA members would result in average net immigration to the UK of 260,000 a year over the next decade.
Home Secretary Priti Patel said: “Under Corbyn’s Labour, immigration would surge and put huge strain on schools and our NHS.
“Jeremy Corbyn has no credible plan for how to deal with the consequences of his open borders policy.
“The biggest risk to our NHS is Corbyn’s plans for uncontrolled and unlimited immigration, forever.”
She added: “His plan to hold another two referendums next year – and all the chaos that will bring – will mean that his Government will not have time to focus on the people’s priorities.
“By contrast, the Conservatives will get Brexit done, end free movement and introduce an Australian-style points system so that we can control our borders and protect our public services.
“We will reduce immigration overall while being more open and flexible to the highly skilled people we need, such as scientists and doctors.
“This can only happen if people vote for a Conservative majority Government so we can leave the EU with a deal.”
Diane Abbott, Labour’s shadow home secretary, said: “This is more fake news from the Conservative Party’s make-believe research department.
“Unlike the Tories, we won’t scapegoat migrants or deport our own Windrush-generation citizens.
“The damage done to our society has been through damaging Conservative cuts to our public services, not by EU nationals coming to work in them.”