The UK believes the Kremlin was behind a hoax call targeting Foreign Secretary Boris Johnson.

Russian pranksters pretended to be the Armenian prime minister in a call to Mr Johnson, during which he discussed the UK's relationship with Moscow and the nerve agent attack on former spy Sergei Skripal and his daughter Yulia.

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

Critics claim 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.

"It is tragic to see a major international power reduced to failed pranks you would usually only see on Trigger Happy TV."

The Guardian reported that during the 18-minute call one of the pair pretended to be Armenian PM Nikol Pashinyan.

Mr Johnson congratulated him 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 ordered the Salisbury nerve agent attack on the Skripals, Mr Johnson said: "We are, like, almost 100pc sure. We will be able to do more to illustrate that.

"I think 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."

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 does not appear to have revealed any sensitive information.

Mr Stolyarov told the Guardian he was impersonating Mr Pashinyan in the call, and said he had also spoken to Europe minister Sir Alan Duncan before the call with Mr Johnson.

A Foreign Office spokesman said: "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."

Downing Street said there would be a "Whitehall investigation" into how the caller was able to get through to the Foreign Secretary.

"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 No 10 spokeswoman said.

Online Editors