WHITEHALL has launched a full investigation into how a “childish” hoax caller was able to target Boris Johnson.

The UK Government believes the Kremlin was behind the 18-minute fake call to the Foreign Secretary, who, among other things, discussed the Salisbury nerve agent attack.

One insider said it was tragic to see a major power like Russia “reduced” to making such pranks.

Downing Street said: “Obviously, this shouldn't have happened. An investigation is under way to determine the circumstances around this call and to make sure that this does not happen again."

A Foreign Office spokesman noted: "The Foreign Secretary 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. These childish actions show the lack of seriousness of the caller and those behind him," he added.

Russian pranksters pretended to be Nikol Pashinyan, the Armenian Prime Minister, in a call to Mr Johnson during which he not only discussed the UK's relationship with Moscow but also the assassination attempt on former spy Sergei Skripal and his daughter Yulia.

Tricksters Alexei Stolyarov and Vladimir Kuznetsov - known as Lexus and Vovan - have claimed a series of high-profile victims, including politicians and Sir Elton John.

Critics say they have links to the Russian security services and a senior UK diplomatic source said: "This seems to be the latest desperate attempt by the Kremlin to save face after it was internationally shamed in the wake of the Skripal attack.

"Boris rumbled them pretty quickly and ended the call,” explained the source, adding: "It is tragic to see a major international power reduced to failed pranks you would usually only see on Trigger Happy TV."

In the call, Mr Johnson congratulated Mr Pashinyan on his "remarkable success" and said: "Obviously, we had hoped to have better relations with Russia than we currently do; I'm afraid Russia seems to be unable to resist malign activity of one kind or another."

He added: "If I have a message to Putin, it's that we don't want a Cold War but we do want to see an improvement in the way Russia behaves."

Asked whether he was sure Vladimir Putin's Russia had ordered the Salisbury nerve agent attack on the Skripals, the Secretary of State replied: "We are, like, almost 100 per cent sure. We will be able to do more to illustrate that.

“It's very important for the Russians to know that, certainly the UK, is absolutely determined to stand firm against them.

"We will continue to tighten the squeeze on some of the oligarchs who surround Putin...you throw a stone in Kensington and you'll find you'll hit an oligarch. Some of them are close to Putin and some of them aren't," he added.

The fact that a prankster was able to speak directly to a Cabinet minister will cause red faces in the Foreign Office and raises concerns about security, particularly given the subject matter of the call; although Mr Johnson did not appear to have revealed any sensitive information.

Later, Mr Stolyarov said he had impersonated Mr Pashinyan in the call and claimed he had also spoken to Sir Alan Duncan, the Europe Minister, before speaking to Mr Johnson.