Emmanuel Macron has stood his ground and insisted Nato seems to be suffering from “brain death” as national leaders gathered for a summit marking the 70th anniversary of the western alliance.

The French President’s remarks were criticised by Donald Trump as “very insulting" to Nato allies.

Sitting alongside Jens Stoltenberg, the alliance’s Secretary General, the US President described Mr Macron’s outburst as a “very nasty statement”.

Noting how France had had a “very rough year” with high unemployment and the yellow vests’ protests, Mr Trump said nonetheless “you just can't go around making statements like that about Nato; it is very disrespectful".

Mr Stoltenberg also defended the alliance, saying: "Nato is active, Nato is agile, Nato is adapting. We have just implemented the largest reinforcement of collective defence since the Cold War."

Mr Macron's criticism followed Turkey's incursion against the Kurds in northern Syria, which alarmed other Nato members because there was no forewarning.

Recep Tayyip Erdogan, the Turkish President, forcefully dismissed Mr Macron's comments, accusing him of "a sick and shallow understanding" of terrorism and suggesting that he was the one who was "brain dead".

Yet, the French President’s concern is shared across many European capitals at the Turkish offensive last October against the Kurds; seen as crucial allies of the West in the fight against Islamic State.

The incursion - which went ahead after Mr Erdogan was effectively given the green light by Mr Trump - was widely regarded as strengthening the position of Russia in Syria while triggering another humanitarian crisis.

There was further alarm among alliance members when Mr Erdogan chose to purchase Russian air defence systems; regarded as a further weakening of his country’s commitment to Nato.

Boris Johnson, who attended a meeting in Downing St with Mr Macron, President Erdogan and Germany’s Angela Merkel to discuss the situation in Syria, is trying to hold the atmosphere together ahead of the formal Nato gathering at a country house hotel in Watford on Wednesday.

Despite allies’ criticism of his outspoken remarks, Mr Macron emerged from a meeting at the US ambassador's residence, to insist there was a need within the alliance for a "strategic clarification" on how to deliver long-term peace in Europe.

He and Mr Trump struck a conciliatory note following their bilateral, both acknowledging Nato needed to look beyond the threat from Russia to issues such as Islamic terrorism.

"The President and I feel that we need more flexibility so we can use it for other things, not just one specific country," explained Mr Trump.

"A lot of people say Nato was originally meant to look at the Soviet Union, now Russia, but we also have other things to look at, whether it is radical Islamic terrorism, whether it is the tremendous growth of China."

Mr Macron acknowledged that there had been US "over-investment" in the alliance for decades; a key complaint of Mr Trump that other Nato members were not paying their way.

As Nato leaders prepared for two receptions, one at Buckingham Palace, and the other in No 10, Downing St sought to instil a sense of unity within the alliance, which it said was the “most enduring and successful in military history” and that Mr Johnson believed it needed to continue to adapt to the evolving threats that it faced.

"It is the cornerstone of Euro-Atlantic security and it helps to keep a billion people safe,” declared a spokesman. "The PM will emphasise that all members must be united behind shared priorities, so Nato can adapt to the challenges ahead," he added.

Wednesday’s meeting is expected to consider new threats, including in the areas of cyber and space, after the alliance last month declared space was one of its operational domains alongside air, land, sea and cyber.