Nicola Sturgeon has undermined the Prime Minister’s attempts to keep Britain in Europe by delivering an extraordinary attack on an “overblown” new Treasury report that warned Brexit could create a deep recession and mass job losses.

The First Minister launched a ferocious assault on the document, which was unveiled only hours earlier by David Cameron and George Osborne, saying the public are too “savvy” to believe its more extreme claims and warning that insulting people’s intelligence could backfire.

Although she insisted she was on the same side as Mr Cameron in wanting a Remain vote, Ms Sturgeon said he should spend the final month before polling day delivering an upbeat message about the EU rather than relying on a “miserable, scaremongering, fear-based campaign.”

Her intervention was warmly welcomed by the Scottish wing of the Vote Leave campaign, which said the SNP leader was correct that “the Treasury claims are overblown and are an attempt to scare the voters.”

It came after Ms Sturgeon was last week accused of spending more time criticising the official Remain campaign than contributing to the cross-party effort to get an In vote on June 23. She has refused to share a platform with senior figures from the Tories, Labour or Liberal Democrats.