Intelligence services investigated unusual activity at the Russian embassy in London in the days before and after the novichok poisoning, it has been reported.

MI5, MI6 and GCHQ looked into unprecedented “frantic comings and goings” at the building in Kensington in the days leading up to the poisoning of the former Russian spy Sergei Skripal and his daughter Yulia, a source with knowledge of the investigation told the Press Association.

The source was quoted as saying: “The intelligence agencies have been investigating unusual and increased activity at the Russian embassy in Kensington in the days leading up to and after the attack on the Skripals.”

“As would be expected, the UK security services have eyes on known and undeclared foreign intelligence operatives,” the source said.

Skripal and his daughter collapsed after coming into contact with novichok at his home in Salisbury on 4 March last year. They recovered, but in July Dawn Sturgess, 44, died when she was exposed to novichok contained in a fake perfume bottle found by her partner, Charlie Rowley.

Britain has accused Russia of being behind the attack. In March last year, Theresa May expelled 23 suspected Russian spies from the London embassy, the largest mass expulsion of diplomats since the cold war.

In a report published ahead of the first anniversary of the Skripal attack on Monday, the Russian embassy said Sturgess had been the “innocent victim of political games”. The 52-page document, entitled Salisbury: Unanswered Questions, the embassy claims there are “inconsistencies in the British narrative” surrounding the events.

“We reiterate our sincere condolences over the tragic death of Dawn Sturgess, who has become an innocent victim of political games,” the report said. “We join her loved ones in aspiring for the full circumstances of what happened to her and others involved to be established.”

In September, Scotland Yard and the Crown Prosecution Service said there was sufficient evidence to charge two Russians, Alexander Petrov and Ruslan Boshirov, with offences including conspiracy to murder after they were caught on CCTV in Salisbury the day before the attack. They are thought to be from Russia’s military intelligence agency, the GRU.

Yulia and Sergei Skripal collapsed after coming into contact with novichok in Salisbury on 4 March 2018. Photograph: Rex/Shutterstock

Russia has repeatedly denied any involvement, with Vladimir Putin claiming the two suspects are civilians. During a TV interview, Petrov and Boshirov said they had been visiting Salisbury to see its famous cathedral.

In February a high-ranking Russian military intelligence service officer, Denis Sergeev, was reported by the website Bellingcat to have been in Britain during the poisoning. He also reportedly visited Bulgaria in the days before a Bulgarian arms trader and his son were attacked with an unidentified poison.

On Friday counter-terrorism officers said they were still trying to find out where the perfume bottle was between the attack in March and the end of June when Rowley found it.

Deputy assistant commissioner Dean Haydon, the senior national coordinator for counter-terrorism policing, said: “A year on from the attacks, there are parts of the picture that we are continuing to piece together, and I am urging anyone who has information that they have not yet passed to police to do so. The information you have could be crucial to securing the prosecution of those responsible.”

The Home Office and the Foreign and Commonwealth Office both declined to comment. Neither the Russian embassy nor GCHQ commented on the Press Association report.