A source on the campaign told the Guardian that No 10 were more interesting in preventing Tory infighting than taking on the big players on the Leave side.
“We were hobbled in what we were allowed to do by No 10. They were sure they were going to win, and their chief interest was putting the Conservative party back together. There was a huge reluctance to attack Boris. A campaign that cannot personalise an attack is fighting with one hand behind its back."
David Cameron reportedly personally vetoed an ad that featured Mr Johnson inside Nigel Farage's pocket - a pastiche of a general election advert featuring Ed Miliband and Alex Salmond.
Another strategist told the paper they had a problem cutting through to voters who did not believe any of the expert warnings about the risks of Brexit.
"The only people the public slightly trusted were Martin Lewis, Richard Branson and, on a good day, Mark Carney so long as he was not seen as a banker. We had a credibility problem, but so now does all public debate in the UK," they said.
Another source said the lack of support for Remain among papers such as The Sun and the Daily Mail had weakened the In campaign's messaging.
"They were sure the economic risk message would bring the voters back to the status quo. Those messages are fine if they are going to be echoed every day in the rightwing press, so creating an echo chamber that the broadcasters have to follow. But the press was never on our side."