French President Emmanuel Macron could use his veto to stop Boris Johnson becoming the general secretary of NATO when he leaves No.10.
Mr Johnson announced he would be resigning as Prime Minister after a number of high profile resignations within the Conservative party.
With both remaining candidates to replace him, Rishi Sunak and Liz Truss, ruling out a government role for Mr Johnson, speculation has increased about what he might do next.
One role mooted for the former Mayor of London is the secretary general of NATO, with Jens Stoltenberg expected to leave the position next year.
Boris Johnson and Emmanuel Macron
Stefan Rousseau
But sceptics believe any attempt to install Mr Johnson into the role would result in Emmanuel Macron using his veto to block it.
A senior Ministry of Defence 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’.”
Ukrainian President Volodymyr Zelenskyy attends a meeting with Prime Minister Boris Johnson
Matt Dunham
Mr Johnson would have to quit his role as Uxbridge MP to take up the role, but sources have claimed he will not stand for it in the next General Election.
The US is believed to favour appointing a British candidate due to their opposition of a potential EU army.
And Oleksii Goncharenko, a Ukrainian MP, told the paper Mr Johnson should take up the role, praising his leadership in the fight against Vladimir Putin.
He 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.”