The government does not have the power to control immigration while we are part of the EU and the prime minister was told this four years ago, his former advisor has told ITV News.
The prime minister's former policy guru Steve Hilton said David Cameron was warned it was "impossible" to meet the government's pledge on immigration if Britain stayed in the EU.
He said civil servants "directly and explicitly" explained that to the Prime Minister four years ago.
But Leave supporter Mr Hilton says Cameron reaffirmed his commitment to target in the 2015 election even though he "had been told it was undeliverable".
Just weeks ago, Cameron insisted he stuck by his ''ambition'' to pull the number of immigrants coming to the UK under 100,000 - but last month net figures topped 333,000.
Mr Hilton told ITV News: "We were told when I was working in government by officials that as long as we were in the EU it would be impossible to deliver the pledge that was made to control the overall level of immigration and bring it down to the tens of thousands.
"That pledge was made again in the 2015 election but the situation hadn't changed. As long as we are in the EU it is not possible for the elected government to control the level of immigration in this country."
Writing in The Daily Mail, Mr Hilton recalled the details of a meeting in the final months of his time as director of strategy.
"In the 2015 Conservative manifesto, the Prime Minister reaffirmed his commitment to the immigration target he had been told was undeliverable.
"When I saw that, I assumed this was either because he was certain he could negotiate a solution within the EU, or was assuming we would leave."
With just 48 hours to go until the referendum vote, Jeremy Corbyn and some of Labour's most senior figures, including former Prime Minister Gordon Brown, are hitting the road in a blitz aimed at winning over wavering party supporters to Remain.
While leading Vote Leave campaigner, Labour's Gisela Stuart and Conservative Andrea Leadsom will take on the leader of the Scottish Conservatives, London mayor Sadiq Khan and TUC General Secretary Frances O'Grady in the BBC's Great Debate.