The former head of MI6 has said Donald Trump lacks the background, experience and instincts of an effective president, as he warned the US may be heading into a new war on the Korean peninsula.

Ex-spy chief Sir John Sawers said Mr Trump was not a person who “fills me with confidence” as he cited increasing tensions with North Korea as a bigger threat to world peace than the conflict in Syria.

But he also said recent events in the Middle Eastern country, which include a US missile strike, do show military and diplomatic “grown-ups” are beginning to steer the US administration, pushing Mr Trump’s ideological staff to the periphery.

His intervention comes ahead of a G7 meeting at which Boris Johnson will lead a diplomatic push for new sanctions to punish Russia for backing the Assad regime, blamed for a deadly chemical weapons attack, and as a US Navy carrier group heads to the Korean Peninsula to ward off missile tests by Kim-Jong Un.

Asked if he is afraid at how Mr Trump may handle the twin crises, Sir John said: “He’s not someone who fills me with confidence.

“He doesn’t have the background and the experience and the instincts of being an effective US president, but it is in our interests that we have a US administration that upholds the international system, that supports its allies and supports international norms.

“What we saw last week [in Syria] was some of the heavyweights, the serious minds in the US administration, H.R. McMaster, the national security advisor, Jim Mattis, the Defence Secretary, and now [Rex] Tillerson this week going to Moscow.

“We see the sensible grown-ups within the administration taking charge and the rather ideological figures around Trump himself being marginalised, and that’s to be welcomed.”

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

It comes amid reports of clashes between Mr Trump’s ideologue alt-right advisor Steve Bannon and others in his administration, most recently over the Syria missile strikes.

Sir John, also former UK permanent representative to the United Nations, told BBC Radio 4’s Today programme he was “no fan” of Mr Trump and that the first three months of his presidency had been “chaotic”, but he said the response to the chemical weapons attack in Syria was “effective”, even if it was not part of a new grand strategy.

He warned however, that it is from North Korea that the potential greatest threat to world peace comes.

The US Navy recently cancelled planned port calls in Australia for the USS Carl Vinson and instead sent the aircraft carrier toward the Korean Peninsula amid concerns over new missile tests.

US military action against North Korea 'an option on the table'

The former spy-chief went on: “If you are looking for a world crisis which could bring about the dangers of a clash between great powers then North Korea is a bigger concern than Syria.

“The move by the Americans to strengthen their forces in the Korean Peninsula, the deployment of this carrier battle group, the demonstration to President Xi in Florida that the US was willing to use force against another state to uphold international order, uphold international norms, this is all part of a move, part of a calculation that North Korea has to be treated very seriously, a very high priority and ultimately it needs a joint US Chinese approach to deal with this unless we are to avoid a further conflict on that peninsula.”