The two accounts underline a challenge Britain is facing as it endeavors to build and maintain an international coalition around the poisoning while keeping much of its evidence secret.
The authorities have released little information about the attack on Mr. Skripal and his daughter, but they have expressed certainty that it was carried out on behalf of the Russian government. Officials have said that Mr. Skripal, a former Russian spy turned double agent who was living peacefully in southwestern England, was very likely targeted as an example to future defectors. Moscow has denied the accusations.
Britain’s public case against Russia has so far rested largely on the nerve agent, an unusual chemical weapon that was developed in the last years of the Soviet Union. But this week, the chief executive of the Defense Science and Technology Laboratory at Porton Down, Britain’s premier chemical weapons laboratory, said its scientists could not identify “the precise source” of the chemical, though it was almost certainly created by a “state actor.”
This undercut statements by Foreign Secretary Boris Johnson that Porton Down scientists had “categorically” traced the agent to Russia. Mr. Johnson swiftly came under criticism from Jeremy Corbyn, the leader of the opposition Labour Party, who said the foreign secretary had “exceeded the information he had been given” and had “egg on his face.”
Diane Abbott, another Labour leader, took aim at Mr. Johnson in an interview with the BBC, saying, “so many people were willing to rush to the media and say it was unequivocally Putin.”
“We will, I hope, get some credit for taking a more thoughtful approach and asking the right questions,” she said.
A new challenge could come within a few days if Viktoria Skripal, who is Mr. Skripal’s niece, follows through on plans to travel to Britain.
Alexander Yakovenko, the Russian ambassador in London, said at a news conference on Thursday that he had offered Ms. Skripal assistance with transport, accommodation and Russian-English translation during her visit.
Viktoria Skripal has repeatedly expressed doubts about the British accusations, suggesting that her relatives could have been sickened by bad fish or attacked by the mother of Yulia Skripal’s boyfriend. She has made several appearances on Russia’s state-controlled television in recent days, in one case appearing alongside the two men accused of poisoning Alexander Litvinenko, another former Russian spy, in London in 2006.
In the conversation broadcast on Russian television on Thursday, Yulia Skripal tells her cousin that she will probably not get a British visa, and that they probably would not be able to see one another.