LONDON/MOSCOW - Twenty three expelled Russian diplomats and their families left London for Moscow on Tuesday as Britain and Russia traded recriminations over a nerve agent attack in England that has plunged relations into their worst crisis since the Cold War.
Prime Minister Theresa May blamed Russia for the attack on a Russian double agent and his daughter - the first known offensive use of a nerve toxin in Europe since World War Two - and gave 23 Russians whom she said were spies working under diplomatic cover one week to leave London.
Russia has repeatedly denied any involvement in the attack on Sergei Skripal and his daughter and, in a reciprocal gesture on Saturday, gave 23 British diplomats a week to leave Moscow as well as closing the British Council in Russia.
As May met top officials and advisers on national security, her spokesman announced new steps to track people coming into Britain who could be deemed a threat to national security.
This was in line with an announcement by May last Wednesday that Britain would draw up new legislation to toughen defences against "hostile state activity".
A state-owned Russian Ilyushin-96 plane, with "Rossiya" and the white, blue and red of the national flag emblazoned on the side, made a special flight from Moscow to London's Stansted airport to pick up the diplomats who were given warm-send off by Russia's top diplomat in London.
Thanking them on behalf of President Vladimir Putin, who on Sunday after his re-election repeated that Moscow had played no part in the attack, ambassador Alexander Yakovenko said: "We are proud of you."
Russia has refused to explain how Novichok, a nerve agent first developed by the Soviet military, was used to strike down Skripal, a former colonel in Russian military intelligence who betrayed dozens of spies to Britain.
Skripal, 66, and his 33-year-old daughter Yulia have been critically ill since they were found unconscious on a bench in the English city of Salisbury on March 4. A British policeman who was also poisoned is in a serious but stable condition.
Russia says it knows nothing about the poisoning and has repeatedly asked Britain to supply a sample of the nerve agent that was used against Skripal.
The United States and European powers say they share Britain's belief that Russia is culpable for the poisoning though they have given no indication of what they will do about it.
British Foreign Secretary Boris Johnson said last week that it was overwhelmingly likely that Putin himself made the decision to use the toxin against Skripal.
The two sides continued to exchange accusations over the affair.
Russian diplomats told the Conference on Disarmament at the United Nations in Geneva that Britain may have produced the toxin itself and that Moscow did not owe any explanations.
Britain's ambassador to the forum, Matthew Rowland, said Russia had given misleading statements on its Novichok programme.
"Instead of engaging on the substantive concern, Russia has sought to confuse the picture with at best misleading procedural arguments," Rowland said. Russia was using "a series of wild hypotheses and half truth and half lies" to deflect attention from the truth, he said.
The Russian foreign ministry has invited foreign envoys to a meeting on March 21 with arms control experts to discuss the affair, the state-run TASS news agency reported on Tuesday.
Putin, who was elected for a fourth term in the Kremlin on Sunday, said Russia had been falsely accused.
"As for the tragedy that you mentioned, I found out about it from the media. The first thing that entered my head was that if it had been a military-grade nerve agent, the people would have died on the spot," Putin told reporters on Sunday.
"Secondly, Russia does not have such (nerve) agents. We destroyed all our chemical weapons under the supervision of international organisations, and we did it first, unlike some of our partners who promised to do it, but unfortunately did not keep their promises," Putin said.
A Cold War-era scientist acknowledged on Tuesday he had helped create the nerve agent, contradicting Moscow's insistence that neither Russia nor the Soviet Union ever had such a programme.
However, Professor Leonid Rink told the RIA news agency that the attack did not look like Moscow's work because Skripal and his daughter had not died immediately.
"It's hard to believe that the Russians were involved, given that all of those caught up in the incident are still alive," he said. "Such outrageous incompetence by the alleged (Russian) spies would have simply been laughable and unacceptable."
Inspectors from the world's chemical weapons watchdog have begun examining the poison used in the attack.