Russia's ambassador to Britain has accused the British authorities of destroying evidence in the Salisbury nerve agent attack.

Alexander Yakovenko said the UK had yet to produce the evidence to back its claims that Russia was responsible for the poisoning of former spy Sergei Skripal and his daughter, Yulia.

"The British government still hasn't produced any evidence in support of its position that would confirm their official version," he told a news conference at the Russian embassy in London.

"We get the impression the British government is deliberately pursuing the policy of destroying all possible evidence."

Mr Yakovenko complained at the continued refusal of the British authorities to grant consular access to Ms Skripal following her discharge from hospital.

He questioned the authenticity of a statement issued by Ms Skripal through the Metropolitan Police saying she did not want to meet Russian officials.

"We are not allowed to see our citizens, talk to doctors, have no idea about the treatment the Russian nationals receive," he said.

"We cannot be sure that Yulia's refusal to see us is genuine. We have every reason to see such actions as the abduction of two Russian nationals.

"The statement made by Scotland Yard on behalf of Yulia Skripal only confirms our suspicions." 

Metropolitan Police Commissioner Cressida Dick, speaking at a briefing at New Scotland Yard, said around 250 detectives were working on the Salisbury investigation.

She said: "It continues - it's a very large investigation. It's large, it's complex it will take time.

"We are of course delighted that one of the people attacked has made a recovery."

Asked if the probe was getting closer to identifying a suspect, she said: "All I can say is we are putting huge effort in, we will continue to do that.

"We will do everything we can to establish the facts, to identify who was responsible and if at all possible, bring them to justice."