Mr Johnson was also prompted by the prankster to discuss Russia
The British Foreign Secretary Boris Johnson held an 18-minute phone call with a prankster, claiming to be the Armenian prime minister.
The pair talked about Russia, Vladimir Putin and the Salisbury poisoning during the conversation, audio released on Thursday revealed.
A UK diplomatic source later hit back, calling it a "desperate attempt by the Kremlin to save face".
Mr Johnson began the conversation by congratulating the hoax caller "very sincerely on your remarkable success", believing he was talking to newly-elected Armenian leader Nikol Pashinyan.
"You can definitely count on the UK and I admire your vision and what you're trying to achieve," he said.
Later in the call, Mr Johnson was prompted to discuss Russia.
He said that "obviously we had hoped to have better relations... than we currently do", adding: "I'm afraid that Russia seems to be unable to resist malign activity of one
kind or another."
Mr Johnson then appeared to laugh when the caller said he was holding a meeting with the Russian president and "hoped he will not poison me with Novichok", the nerve agent used on Sergei and Yulia Skripal.
"The only thing the Russians respond to is determination and firmness," was Mr Johnson's advice.
It is not clear when the call took place, but the real Armenian prime minister held the talk with Mr Putin referred to on May 14th - 10 days ago.
It comes on the same day the investigation into the downing of Malaysia Airlines flight MH17 blamed Russia, and the day after Yulia Skripal gave her first interview.
During the phone call, Mr Johnson also said the UK was "like almost 100% sure" that Russia was behind the Salisbury poisoning and promised: "We will be able to do more to illustrate that...
"We will continue to tighten the squeeze on some of the oligarchs who surround Putin.
"You throw a stone in Kensington and you'll hit an oligarch. But some of them are close to Putin and some of them aren't."
Mr Johnson put the phone down when the prankster claimed that the prime minister of Ukraine had recently been poisoned, but recovered after taking an antidote "he keeps with him always".
A British Foreign Office spokesperson 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."