A recording of the call, said to have taken place last week, was posted on YouTube.

In the recording, Johnson says Britain is "almost 100 per cent sure" President Vladimir Putin's Kremlin ordered the attack on Skripal and his daughter Yulia. Moscow denies involvement in the incident, which has sparked a crisis in UK-Russia relations.

Johnson is also heard lamenting the poor state of UK relations with Russia, saying Moscow seems "unable to resist malign activity of one kind or another."

The Foreign Office said Johnson "realised it was a hoax, and ended the call. We checked it out and knew immediately it was a prank call."

"The use of chemical weapons in Salisbury and Syria and recent events in Armenia are serious matters," it said in a statement. "These childish actions show the lack of seriousness of the caller and those behind him."

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Prime Minister Theresa May's office said "obviously this shouldn't have happened," and announced there would be a government investigation into how the hoaxers got through to Johnson.

Britain suspects the comedy duo have backing from the Kremlin.

Stolyarov and Kuznetsov, who have fooled high-profile victims around the world, have denied links to Russia's security services. In 2015, they phoned Elton John pretending to be Russian President Vladimir Putin after the musician criticised Russia's stance on gay rights.

AP with Telegraph, London