The election of Donald Trump could cast into doubt the future of Nato and strengthen the hand of Vladimir Putin's regime in Moscow, security experts have warned.

The Republican's upset victory in the US election has provoked unease in Europe, where his lukewarm comments on the transatlantic nuclear alliance during the campaign were met with sharp criticism.

Former RAF chief Sir Michael Graydon suggested Trump's victory could be a "wake-up call" to European nations currently "freeloading" on American and British defence spending.

Meanwhile, former head of the army General Lord Dannatt called for an increase in UK defence spending.

During the election campaign, president-elect Trump suggested his administration would only aid allies "if they fulfil their obligations to us".

This raised questions about the integrity of Nato's Article Five, the instrument which binds members to common defence.

The effect of the article means an attack on one member state is considered an attack on all.

Sir Michael told BBC Radio Four's Today programme: "I'm concerned about what he has said in regard to Nato.

"And if you start off from the position that Mr Putin would love to see Nato disintegrate, then some of the things Mr Trump has said takes us quite a long way along that track.

"I think he has hinted that he actually doesn't believe in Nato. So, those are the sort of things that give a lot of hope to Mr Putin."

He added: "And I think a strong statement from him saying he supports Nato, but 'you've got to pay your fair share' would be a rather more helpful approach.

"I think the key to all of this will be if he surrounds himself with good advisers. I think he will find that it is a lot more difficult in a very complex world to do some of the things that he wants to do, and that working with allies will be much better than standing and going back into an isolationist America which is one do the implications of the things he has been saying.

"Russia's already pretty encouraged with the uncertainty that exists in Nato at the moment."

General Lord Dannatt told the Daily Mail: "We've seen Donald Trump threaten all sorts of things, we should take that threat seriously.

"From a UK point of view we have to think about increasing our own defence expenditure.

"We should actually realise that Trump means it and states have to do more about their own role in Nato.

"If the US walks away, Nato ceases to exist, almost by definition, so we would have to invent something else. In the context of Brexit that is ridiculous."