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.