Intelligence services are understood to have investigated increased and unusual activity at the Russian embassy in London in the days before and after the Novichok poisoning.
MI5, MI6 and GCHQ looked into frantic comings and goings at the building in Kensington in the days leading up to the moment former Russian spy Sergei Skripal and his daughter Yulia came into contact with the nerve agent and after they were found unconscious, according to a source.
In the wake of the attack, which took place in Salisbury a year ago on Monday, the observations were deemed to be significant and of interest, it is understood.
A security source told the Press Association: 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.
The source, who has knowledge of the investigation, said lines of inquiry included looking at movements- described as frantic and unprecedented which were observed at the time, adding: As would be expected, the UK security services have eyes on known and undeclared foreign intelligence operatives.
Britain has accused Russia of being behind the attack.
Intelligence services continue to believe it was ordered at the highest level in the Russian state, the source said.
Some two weeks after the attack, Prime Minister Theresa May kicked out 23 suspected Russian spies from the London embassy in the largest mass expulsion of diplomats since the Cold War.
Both Mrs May and Home Secretary Sajid Javid claimed the Russian government had been involved, saying approval came from a senior level in Moscow.
A fortnight after the attack, the Russian embassy in the UK’s official Twitter account posted a comment which said: In absence of evidence, we definitely need Poirot in Salisbury! with an accompanying picture of actor David Suchet in costume as his famed character.
A week later, Russia’s ambassador in London was ridiculed for claiming Britain was behind the attack.
Following a press conference at the embassy, Alexander Yakovenko was said to have been called a Russian Comical Ali by Conservative MPs, after Saddam Hussein’s spokesman Mohammed Saeed al-Sahhaf who earned the nickname over his claims.
The Home Office and the Foreign and Commonwealth Office both declined to comment.