Jeremy Corbyn could pass classified US intelligence to Russia and Iran if he becomes prime minister, Donald Trump has been warned.

A report, backed by former advisers to the US president, urges Mr Trump to consider withholding sensitive information and even pushing for the country’s Nato membership to be “downgraded” if the Labour leader enters Downing Street.

The analysis, by a Washington DC think tank, highlights the support displayed for Mr Putin by some of Mr Corbyn’s advisers and claims the UK will “cease to become a reliable partner” under his leadership.

It cites Mr Corbyn’s response to the Salisbury attack, as part of which he demanded evidence of Kremlin involvement, and his spokesman suggested any such intelligence could be unreliable because of failures over weapons of mass destruction in Iraq.

The report highlights mounting fears about a Corbyn premiership, amid the likelihood of a general election in the coming months. This week, a ComRes poll for The Telegraph put Labour a point behind the Tories.