However, there was concern among the top brass, with Lord Dannatt, former head of the British Army, saying that he could not support Mr Johnson because of his character.
The peer told The Telegraph: “Undoubtedly he has done a lot of good things and our full square support for Ukraine is fantastic.
“But I am afraid it is the personal stuff, the lack of integrity, the lack of trust. Frankly, we don’t want to expose Boris Johnson on the international stage for more ridicule. He is a national embarrassment.”
A senior Ministry of Defence figure also expressed doubts, given there was a likelihood that Emmanuel Macron, the French President, would veto him.
One source told The Telegraph: “The reality is that Nato sec gens are appointed by unanimous decision. Any country can veto.
“Do you think President Macron would nominate Boris Johnson to be the sec gen of Nato? It is a challenge for a Brit full stop. You need the United States to support you and the French to say ‘yes’.”
The next head of Nato was due to be appointed this September but was delayed by a year due to the crisis in Ukraine.
The last UK politician to become Nato secretary general, one of the world’s most senior defence appointments, was Lord Robertson of Port Ellen – who at the time was Labour Defence Secretary – in 1999.
‘He understands the challenges’
Oleksii Goncharenko, a Ukrainian MP, was at the forefront of calls for Mr Johnson to take up the post when present secretary general Jens Stoltenberg steps down next year
Mr Goncharenko said: “Boris Johnson’s leadership during this war has ensured vital military, economic and humanitarian supplies have been provided to Ukraine.
“The UK has been one of our most important supporters and has imposed meaningful sanctions on Russia.
“He would be the right person to take over from Jens Stoltenberg as he understands the challenges that Nato and the West face.
“He will always support Ukraine in our fight against the 21st century’s Hitler and will support Nato members and Nato goals with the same level of unstinting support.”
Number 10 declined to comment.