An ex-Nato Secretary General has demanded European nations spend more on defence, warning that Donald Trump’s threatened withdrawal of US forces would see Vladimir Putin’s Russia “advance”.

Anders Fogh Rasmussen said it was inevitable Mr Putin would adopt a more “assertive” approach in Eastern Europe if there is any US drawback and urged EU leaders to meet their commitment of spending 2 per cent of GDP on defence.

It comes after President-elect Trump threatened, while on the campaign trail, to withdraw support for nations enjoying US defence capabilities unless they paid their way.

Mr Rasmussen urged Nato to send a signal soon that America’s commitment to defending allies “is unchanged”.

Speaking to the BBC, he said “the Europeans should pay more”, before adding: “We have five out of 28 allies that fulfil the 2 per cent criteria right now, including the United States ... we know that a lot of countries, not least in the East, have now decided to increase their investment and reach the 2 per cent within the next three, four, five years. "

He went on: “If the United States were to withdraw from Europe, then without any doubt Russia would advance, and just exercise a more assertive behaviour in the East and that would weaken European security.”

It comes after current Nato Secretary-General Jens Stoltenberg said Europe and the US must stick together to face what are the greatest security dangers for a generation.

Mr Rasmussen poured cold water on the idea of a European army, which so far includes plans to set up a new EU defence headquarters.

He added: “What is needed now is more European concrete investment in defence capabilities, we don’t need new headquarters.

“While I was Secretary General of Nato we reduced the number of headquarters.