Russia's Ambassador to Britain Alexander Yakovenko suggested that former double-spy Sergei Skripal and his daughter Yulia may have been intentionally injected nerve agent just before their blood samples were taken for investigation by an international chemical weapons watchdog.

Yakovenko was briefing international media in London on Friday, seven weeks after the poisoning of the Skripals on March 4 in the English city of Salisbury.

Britain and some other countries have blamed Russia for the act, but Russia denies any involvement.

Britain's own investigation discovered that Skripals were poisoned with a military-grade nerve agent from the Novichok group, which the UK claims that Russia holds a small stockpile of.

Last week the international chemical weapons watchdog Organization for the Prohibition of Chemical Weapons confirmed the Britain's findings of the identity of the toxic chemical. But it did not assign the blame to any country.

Yakovenko said an OPCW report issued to Russia subsequently to the investigation causes concerns.

He said the OPCW report shows some samples contained pure nerve agent with little sign of decomposition, which raises alarm because nerve agents normally would be expected to react with naturally-occurring chemicals inside the body.

"This is strange given that 18 days have passed between the poisoning and the arrival of the OPCW team to the UK. This might mean that the chemical was intentionally injected just before the samples were taken."

Yakovenko said he does not want to blame to the British government for what he suspects to be an intentional injection of chemicals in the absence of evidence, but urged transparency in further investigation.

Yakovenko's team has requested that the British government allow Russian specialists to carry out medical examination of Yulia.

Yulia has been released but her father remains in hospital. In a statement released by the British police, Yulia rejected overtures from Russian officials who want to speak to her.

But Yakovenko said Russia cannot be sure that Yulia's rejection is genuine, and repeated Russia's demands to see the Skripals. He said the UK's failure to grant access meant the case should now be seen as "an abduction" of two Russian citizens.

In another development, The Telegraph reported that British police and intelligence agencies have identified key suspects in the poisoning attack. It reported that counter-terrorism police are now trying to build a case against "persons of interest" who are believed have already gone back to Russia.

Skripal has lived in Britain since he was part of a spy swap in 2010.

Last Friday UK National Security Adviser Mark Sedwill explained the UK's suspicion that the Russian state was responsible for the Salisbury attack in a letter to Nato Secretary General Jens Stoltenberg.

The letter cited several pieces of evidence, including Britain's belief that Russian intelligence agencies had been monitoring the emails of Yulia as early as 2013, and that Britain believes Russia holds a small stockpile of Novichok, and that Russia has a track record of conducting state-sponsored assassinations.

"It is highly likely that the Russian state was responsible for the Salisbury attack. Only Russia has the technical means, operational experience and the motive," Sedwell wrote.

Meanwhile, this week the British government started a process to decontaminate toxic hotspots in Salisbury, in nine areas identified by government scientists to be at risk. The multi-million pound operation to decontaminate the locations is expected to take months.

The British government said Salisbury remains "safe for residents and visitors" and extra precautions need not be taken.

Contact with the writer at cecily.liu@mail.chinadailyuk.co.