BORIS Johnson has been caught up in an embarrassing security breach, after he discussed the UK's relationship with Russia in a phone conversation with pranksters posing as the Armenian prime minister.

The Foreign Secretary spoke about subjects including the poisoning of former spy Sergei Skripal and his daughter Yulia during the hoax call.

Pranksters Alexei Stolyarov and Vladimir Kuznetsov, known by their prankster monikers Lexus and Vovan, have claimed a series of high-profile victims including various politicians and the singer Sir Elton John.

The Guardian obtained what the pair claimed to be a recording of the 18-minute call in which one of them pretended to be Armenian Prime Minister Nikol Pashinyan.

Johnson congratulated the "prime minister" on his "remarkable success" and added: "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 that "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, Johnson said: "We are like almost 100% 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 Johnson will cause red faces in the Foreign Office and raises concerns about security, particularly given the subject matter of the call which the hoaxers said took place this week.

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."

Critics claim the pranksters 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."

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