British investigators will update an emergency response committee of senior ministers on Wednesday about a mystery substance that struck down a former Russian double agent and his daughter.
Sergei Skripal, once a colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench outside a shopping centre in the southern English city of Salisbury on Sunday afternoon.
Both remain critically ill in intensive care.
"The focus at this time is to establish what has caused these people to become critically ill," Assistant Commissioner Mark Rowley, Britain's top counter-terrorism officer, said.
"This investigation is at the early stages and any speculation is unhelpful at this time."
Counter-terrorism police are leading the investigation and Britain's military research laboratory at Porton Down is trying to identify the substance which caused Skripal, 66, and his daughter to collapse.
The suspected poisoning prompted Foreign Secretary Boris Johnson to say that if Russia were behind the incident then Britain may not participate "in the normal way" in this summer's football World Cup in Russia.
Russia denied any involvement, scolded Mr Johnson for "wild" comments and said anti-Russian hysteria was being whipped up intentionally to damage relations with London.
A source close to the investigation said Russian involvement in the Skripal poisoning was just one of the versions being looked at by counter-terrorism investigators with assistance from the MI5 domestic intelligence agency.
Police said new cordons had been added near Solstice Park, a business park, in the nearby town of Amesbury. They have sealed off the area of Salisbury where Mr Skripal was found as well as the Zizzi pizza restaurant where they dined and the Bishop's Mill pub where they had a drink.
Some emergency workers were treated after the incident and one remains in hospital.
The British capital has been dubbed 'Londongrad' due to the large amounts of Russian wealth which have flowed westwards since the 1991 fall of the Soviet Union. It is the Western city of choice for many oligarchs from the former Soviet Union.
Britain has specifically drawn parallels with the 2006 murder of ex-KGB agent Alexander Litvinenko who was killed with radioactive polonium-210 in London.
Scotland Yard, which took over the investigation from Wiltshire Police on Tuesday, said it was keeping an "open mind" and the case had not been declared a terrorist incident.