General Sir Richard Barrons said President Trump had inherited a "win-lose" attitude from the business world and his military and political advisors might not be able to pull him back from doing something "mad" that would provoke a war.

The warning came a day after the Bulletin of the Atomic Scientists moved its 'Doomsday Clock' 30 seconds closer to midnight, in response to Mr Trump's statements on climate change, expanding the US nuclear arsenal and the questioning of intelligence agencies.

It is the closest the clock has been to midnight in more than 60 years, since the hydrogen bomb tests by the US and USSR.

Sir Richard was one of the most respected military leaders in the UK: he was commander of the Joint Forces Command from 2013 until his retirement in April 2016 and is friends with the Trump administration's new head of homeland security, retired Marine General John Kelly, and General James 'Mad Dog' Mattis, the new secretary of defence.

Sir Richard told a meeting of the Association of European Journalists in London on Friday that he was very concerned about Mr Trump's "win-lose" mentality.

Asked how he thought Mr Trump's approach to business would translate to geopolitics, Sir Richard replied "I think badly".

"In my tiny and unpleasant exposure to the commercial world it seems to me you go into a negotiation … and you know you can walk away. But in geopolitics there's no walking away," he said.

He did not subscribe to the view that Mr Trump as president would be different to Mr Trump the businessman, he said.

"If Mr Trump is confronted – given the nature of his approach to his life so far – he may respond in a way that is psychologically normal for him but deeply dangerous for the rest of us," Sir Richard said.

He imagined a scenario in the South China Sea – where, he said, the Chinese military was "ambitious, very face-conscious, it's getting better equipment, it's ranging further … it's basically putting its balls on the fence (but) it's very inexperienced".

If there was a confrontation and a Chinese frigate commander again 'paints' an American aircraft with its fire control radar, then that would be an offensive act, Sir Richard said.

"What if that American commander isn't so confident that he just chins it off, if he launches his missile in response, which under self-defence he's entitled to do? And sinks a Chinese ship? Where does that take us?

"Wars generally start for really bad reasons and the red mist descends and you lose control … I think the risk of that is evident."

He said he did not know how much Gen Kelly, Gen Mattis and others in Congress would rein in the president if he did something "mad".

Sir Richard said he, Gen Kelly, Gen Mattis, and Mr Trump's national-security adviser, the retired lieutenant general Michael Flynn, were "all friends" from their time in Iraq and Afghanistan.

"Particularly John Kelly and Jim Mattis are first-rate military warriors who understand how to operate at a strategic level," he said. "They understand the interface between the military, politics and policy and are steeped in hard campaigning.

"(But) what they are not is politicians in any way and they've gone into political jobs … if their jobs come down to using their judgment and experience that they have, they'll provide superb advice. But if they're dragged into a political way of doing business, this will be uncharted waters."

He said he had no sense yet how they would forge a relationship with the Trump administration.

"I don't think the Trump administration knows yet what it's going to do," Sir Richard said. "There's got to be a very interesting discussion between Mr Mattis and others and Mr Trump on their approach to collective security.

"And then I think they've got to have an as-interesting discussion about whether they want to ramp up US defence spending and what they actually want to spend that money on."