Russia's ambassador to the U.K. says strained relations could force the communist nation to take its most "extreme measure" against the NATO member: severing diplomatic ties.

The relationship between the nations is "undoubtedly at a very low level" and has been getting worse for years, Amb. Andrei Kelin said in an interview Monday in the pro-Kremlin newspaper Izvestia, according to Newsweek.

The U.K. could find itself the first member of NATO with which Russia severs diplomatic ties, he said.

Relations began to sour in 2018 after former spy Sergei Skripal was poisoned in Britain, Kelin said. Skripal was a former Russian intelligence officer who was a double agent for the U.K. He was poisoned with a banned nerve agent.

Still, Kelin said, the two countries continued business relations until Feb. 24 of this year — the same day that Russia invaded Ukraine.

"Then … London curtailed almost all relations, except for bilateral diplomatic ones," he said. "Severing diplomatic relations is the most extreme measure, which already includes military relations."

Neither country wants to end diplomatic relations at this time, Kelin said.

Whether bilateral ties can be reestablished will depend on the war on Ukraine and "where it all comes to," he added.

"We had periods of good relations with London — and not only after the First World War and during the Second World War," he said, pointing out that Russian President Vladimir Putin visited the U.K. twice in the mid-'90s and early 2000s, and that Queen Elizabeth II visited St. Petersburg and Moscow.

"Then we built really good relations with the U.K.," he said. "During the London Olympics, there was even a Russian pavilion near Kensington Palace. That is, everything was in our relationship. And even now it is impossible to say that everything is over — it's time to put an end to it. I do not believe in this."

Evidence the relationship is not strong: British Prime Minister Rishi Sunak slammed Russia's invasion of Ukraine at November's G20 summit in Bali.

"It is notable that Putin didn't feel able to join us here. Maybe if he had, we could get on with sorting things out," Sunak said. "The single biggest difference that anyone could make is for Russia to get out of Ukraine and end this barbaric war."

And Putin did not congratulate Sunak when he was elected prime minister in October.

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