Downing Street is “panicked” over the EU referendum amid growing internal signs that support for the Leave campaign is surging.

Private data and internal polling which shows that after two weeks of building momentum there have been huge swings to the Brexit campaign has prompted growing alarm in the Remain camp, sources have said.

It has led to a marked change in strategy, with the campaign to keep Britain in the EU now orchestrating a series of highly personal attacks on Boris Johnson, one of the leaders of the Leave campaign.