Keir Starmer to rage at Reform UK and Brexit in front of world leaders in major address

He will brand both Zack Polanski and Nigel Farage's parties "the peddlers of easy answers on the extreme left and the extreme right".

"It's striking that the different ends of the spectrum share so much. Soft on Russia and weak on Nato - if not outright opposed," the PM will blast.

"And determined to sacrifice the longstanding relationships that we want and need to build, on the altar of their ideology.

"The future they offer is one of division and then capitulation. The lamps would go out across Europe once again. But we will not let that happen."

In 1914, then-Foreign Secretary Sir Edward Grey issued the same warning the day before Britain entered the First World War.

Sir Keir will also turn his fire on Brexit in a bid to sway European leaders to back him in his push to back away from "overdependence" on the US.

"We are not the Britain of the Brexit years any more," he will say.

"Because we know that, in dangerous times, we would not take control by turning inward. We would surrender it. And I won’t let that happen.

"There is no British security without Europe and no European security without Britain. That is the lesson of history - and it is today's reality too."

In the face of Americans including Secretary of State Marco Rubio in the audience, Sir Keir will explicitly demand a more "European Nato".

He will also call for Europe to drive towards sovereign deterrence and hard power - a major move away from the "special relationship" years of tight-knit cooperation on US military and joint defence projects.

The Defence Secretary is also in Munich, where he has suggested France and Germany could join the UK's Tempest fighter jet project, also known as GCAP.

That will play into American hands, with US diplomats pressuring Europeans to work more closely together on defence to save money.

The States are thought to have grown increasingly weary of inaction on the Continent.

Just days ago, it emerged that Donald Trump was pushing Volodymyr Zelensky to hold an election and possibly bring an end to the war in Ukraine so he could turn his focus to domestic politics ahead of the midterms.

Back in Britain, Reform and the Greens launched fierce slap-downs of the PM's upcoming remarks.

A Reform spokesman said: "This is a speech from a weak Prime Minister on the verge of being hounded out of office by his own party.

"This is a man that refuses to find the money to increase defence spending and is making our country weaker and less secure.

"Reform UK believes our priority should be rebuilding our armed forces, properly funding defence to at least 3.5 per cent of GDP, standing up to China and Russia, and strengthening our bilateral relationships."

While a Green source told The Times Sir Keir was a "caretaker Prime Minister running scared, losing what’s left of his authority by going abroad to a summit on our future security and making cheap smears against the Green Party".