Russia's ambassador to the UK has suggested that Sergei and Yulia Skripal may have been injected by British authorities with nerve agent produced at Porton Down.
Alexander Yakovenko repeated Russia's demands to see the former spy and his daughter, claiming that the UK's failure to grant access meant the case should now be seen as "an abduction of two Russian citizens".
The comments came in a press conference at which Mr Yakovenko accused Britain of lying not only over the Skripal case, but also about the chemical weapons attack in Syria and alleged Russian cyber attacks on the UK.
They represent the latest in a series of widely-varying and sometimes contradictory explanations produced by Moscow for the events in Salisbury, which have been dismissed by the Foreign Office as an attempt to distract attention from the UK's conclusion that Russia is "very likely" to be responsible for poisoning the Skripals.
A report by experts at the Organisation for the Prohibition of Chemical Weapons this week confirmed the UK's analysis that the attack on the Skripals in Salisbury on March 4 was conducted with the nerve agent Novichok.
But Mr Yakovenko said that details of the OPCW report gave Russian experts cause for concern.
The OPCW reported that the Novichok contained in blood samples from the victims showed little sign of decomposition, even though the agent would normally be expected to react with naturally-occurring chemicals inside the body, he said.
"This is strange, given that 18 days passed between the poisoning and the arrival of the OPCW in the UK," said Mr Yakovenko.
"It might mean that the chemical was intentionally injected just before the blood samples were taken."
The fact that the OPCW found the sample of Novichok to be "of high purity" suggested that it was synthesised in a laboratory rather than an industrial plant, he said.
And he claimed that the speed with which the chemical was identified after the attack suggested the UK already had its own samples.
Referring to the Government's defence research establishment at Porton Down, he said: "We know what kind of laboratory is only 10 miles from Salisbury."
Mr Yakovenko challenged the independence of the OPCW investigation, saying that it had been arranged on a bilateral basis with the UK rather than under the standard practices set out in the Chemical Weapons Convention.
"The work of the OPCW experts was conducted under the control of the British side," he said.
"Pressure on them can't be ruled out.
"They checked only the sites designated by the UK beforehand and they looked only for the substance identified by the UK.
"The format chosen cannot guarantee impartiality and the comprehensive nature of the verification."
Russia would not take the report's conclusions at face value unless it was given full access to the victims and to the materials gathered in the investigation, he said.
Mr Yakovenko said Russia was acting in accordance with international law in wanting to speak to its citizen Yulia Skripal, who is now said to have left hospital and be recovering.
He added: "We want to see that she is in good shape and good health. She can tell us herself that she doesn't want our help."
He repeated allegations that the decontamination operations under way in Salisbury amounted to an attempt by the UK authorities to destroy evidence relating to the attack.