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LONDON — Britain's foreign minister said Sunday that he has evidence Russia has been stockpiling a nerve agent in violation of international law, after a Russian envoy suggested the toxin used to poison a former spy in England could have come from a U.K. lab.

Foreign Secretary Boris Johnson said the trail of blame for the poisoning of Sergei Skripal and his daughter Yulia "leads inexorably to the Kremlin."

Johnson told the BBC that "we actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination but has also been creating and stockpiling Novichok" — the type of nerve agent Britain says was used in the attack.

Britain and Russia have each expelled 23 diplomats, broken off high-level contacts and taken other punitive steps in the escalating tit-for-tat dispute, which clouded the run-up to Sunday's presidential election in Russia. President Vladimir Putin is expected to win a fourth term, amid widespread voter apathy.

Western powers see the poisoning of the Skripals as the latest sign of increasingly aggressive Russian interference in foreign countries.

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Johnson said he will brief European Union foreign ministers on the case Monday before meeting NATO Secretary-General Jens Stoltenberg.

The foreign secretary said officials from the Netherlands-based Organization for the Prohibition of Chemical Weapons would also arrive in Britain on Monday to take samples of the nerve agent used to poison the Skripals.

Britain says it is Novichok, a class of powerful nerve agent developed in the Soviet Union toward the end of the Cold War. Tests to independently verify the British findings are expected to take at least two weeks, Britain's Foreign Office said.

Vladimir Chizhov, Moscow's EU ambassador, said Russia has no chemical weapons stockpiles and was not behind the poisoning.

"Russia had nothing to do with it," Chizhov told the BBC.

Chizhov pointed out that the U.K. chemical weapons research facility, Porton Down, is only eight miles from Salisbury, where Skripal — a former Russian intelligence officer convicted in his home country of spying for Britain— and his daughter were found March 4. They remain in critical condition.

Asked whether he was saying Porton Down was responsible, Chizhov replied: "I don't know."

The British government dismissed the ambassador's suggestion as "nonsense."

Britain's Foreign Secretary Boris Johnson speaks on one of the BBC's flagship political shows, Britain March 18, 2018. Reuters

Johnson said it was "not the response of a country that really believed itself to be innocent."

Johnson said Britain's National Security Council will meet this week to discuss "what further measures if any" the country will take.

Opposition lawmakers are calling on the government to clamp down on the illicitly gained money of wealthy Russians in Britain. Critics say U.K. authorities have been slow to investigate the origins of the wealth invested in London's financial district and property market.

Russia's ambassador in London, Alexander Yakovenko, called for "cooler heads," telling the Mail on Sunday that the dispute is "escalating dangerously and out of proportion."