George Osborne has admitted the Remain campaign lacked "authenticity" and "optimism", and suggested its focus on the economy helped alienate voters.
Speaking on the BBC's Andrew Marr Show, the former chancellor said he had not been a "great fan" of holding a referendum on EU membership, but had done "everything I could" to try to persuade voters.
He stopped short of saying that what became dubbed 'Project fear' was a failure, telling Mr Marr: "That's your characterisation."
But speaking about the campaign to keep Britain in the EU, he acknowledged it had "lacked some of the authenticity [and] some of the optimism of the Leave campaign".
"We discovered there was not much out there in terms of support for European friendship," he said. "So we ended up talking a lot about the economy."
As chancellor Mr Osborne had been one of the Remain campaign's main spokespeople, arguing that the economy would be severely damaged if the country voted to pull out of the EU.
But while in the immediate aftermath of the June Brexit vote stock markets wobbled and the currency fell, many of Mr Osborne's dire predictions have failed to come true.
Also in his interview Mr Osborne indicated he would favour a soft Brexit, saying there should be no 'red lines' in negotiations over Brexit, other than leaving the EU.
"I am certainly someone who believes that it is in Britain's national interest ... that we have the closest possible relations with key partners of ours like the French and the Germans and the Dutch."
On immigration, he said: "There are concerns about immigration which can be addressed ... [but] let's not lose the massive contribution that immigrants have made over many centuries, and continue to make, to Britain's prosperity and society."