British Prime Minister Theresa May on Monday repeated her belief that Russian Federation was responsible.
Asked about indications of the origin or type of substance used in the March 4 attack on Sergei Skripal and the former Russian spy's daughter in the English city, he said he "cannot project the outcome of such technical work".
They will also meet police before sending samples to worldwide laboratories for testing, with results expected to take at least two weeks, according to British officials.
Russia's destruction of its chemical weapons after the fall of the Soviet Union had been verified, and Russian Federation had not conducted any research into Novichok although research continued in Britain and possibly in many other countries, he said.
Russian diplomats and children disembarked but have not left the embassy yet, the photographer said.
Moscow is awaiting almost two dozen Russian diplomats ordered to leave Britain following a nerve agent attack on United Kingdom soil. Alexander Yakovenko, the ambassador, thanked them on behalf of President Putin, telling them: "We are proud of you". They remain in critical condition.
On Sunday, Britain's Johnson accused Moscow of stockpiling the nerve agent over the last ten years in violation of the worldwide Chemical Weapons Convention.
He claims Britain's use of the name Novichok for the nerve agent is meant to convince the public that Russian Federation is to blame.
Prime Minister Theresa May is expected to announce more sanctions against Russian Federation after a meeting of the National Security Council. Western powers have described it as a sign of increasingly aggressive Russian meddling overseas.
They, however, cautioned that the full investigation may take months to complete. The focus is on the movement of the Skripals in the hours before they were found unconscious.
After the first known offensive use of such a nerve agent on European soil since World War Two, May last Wednesday gave 23 Russians she said were spies working under diplomatic cover at the embassy a week to leave.
The Kremlin, for its part, has called the allegations "groundless" and has announced it will expell 23 British diplomats and close the British Consulate in St. Petersburg in turn.
Investigators from the global Organisation for the Prohibition of Chemical Weapons (OPCW) arrived in Britain on Monday to collect samples of the nerve agent used.