MOSCOW — Nearly two dozen Russian diplomats expelled by Britain over the poisoning of an ex-spy arrived home Tuesday, while a scientist involved in the creation of the nerve agent said it could be manufactured by other countries.

Former Russian double agent Sergei Skripal and his daughter, Yulia, are in critical condition in the English city of Salisbury after being poisoned on March 4. Britain said they were poisoned with a class of nerve agent known as Novichok and blamed Russia for the attack.

Russian scientist Leonid Rink told the state RIA Novosti news agency that Britain and others could easily synthesize Novichok after chemical expert Vil Mirzayanov immigrated to the United States and revealed its formula.

‘‘It’s easily available to professionals, and there is no problem for Britain, the US, and other developed nations to create such a weapon,’’ he said.

Britain has dismissed claims the nerve agent could come from the United Kingdom. On Sunday, Russia’s ambassador to the European Union suggested the nerve agent could have come from Britain’s chemical weapons research facility, Porton Down. The British government said that was ‘‘nonsense.’’

On March 14, British Prime Minister Theresa May gave the 23 diplomats — whom she said were undeclared intelligence agents — a week to leave Britain. Russia responded by expelling the same number of British diplomats, who are expected to leave in the coming days.

Russia has fiercely denied any involvement, saying that it had no motive to kill Skripal, who was convicted of spying for Britain but released in a 2010 spy swap. It said that it had completed the destruction of its chemical arsenals last year under international oversight.

White House spokeswoman Sarah Huckabee Sanders said the poisoning didn’t come up in President Trump’s phone call Tuesday with Russian President Vladimir Putin.

Putin’s spokesman, Dmitry Peskov, dismissed demands by Britain and its allies that Russia prove its innocence, saying that it’s Britain who must provide proof.

‘‘Let’s stay sober-minded and first of all wait for proof from Britain’’ that Russia was to blame, he told reporters.

Rink said Britain has refused to provide a sample of the agent it said was used in the poisoning because tests would reveal that it hadn’t come from Russia. He said each lab has its own chemical ‘‘signature,’’ allowing experts to trace its origin. ‘‘It would be immediately clear that it wasn’t ‘cooked’ in Russia,’’ he said.

Britain said experts from the Organization for the Prohibition of Chemical Weapons are taking samples of the nerve agent, which will be tested in international laboratories.