Interior minister Amber Rudd will chair a meeting of Cobra, which is called to deal with national emergencies, to assess the situation of Sergei Skripal, who moved to Britain in a 2010 spy swap.
He and his daughter Yulia are in critical condition in hospital after being exposed to an unknown substance, which led them to collapse on a bench in the southwestern English city of Salisbury on Sunday.
Police say they are keeping an open mind about what happened, but Foreign Secretary Boris Johnson on Tuesday pointed the finger at Russia.
He noted the "echoes" with the 2006 poisoning in London of former Russian spy and Kremlin critic Alexander Litvinenko, which Britain has blamed on Moscow.
Authorities have yet to publicly identify the source of the Salisbury poisoning, but experts said it was unlikely to be radiation as in Litvinenko's case.
Skripal, 66, and his 33-year-old daughter had lunch at a nearby restaurant before walking to the shopping center, where witnesses said they appeared "out of it."
"Radiation poisoning tends to take tens of hours to several days to show symptoms after exposure," said Professor Malcolm Sperrin, a medical physics expert with the National Health Service.
"This may have been chemical, but we can't be sure."
Police reportedly took away the table and chairs where Skripal and his daughter were sitting in Zizzi's restaurant for analysis.
Some emergency services personnel who treated the pair required medical treatment, and The Sun tabloid reported that two police officers had itchy eyes, wheezing and rashes.
Moscow on Wednesday accused British politicians and journalists of using the suspected poisoning of a former Russian spy in Britain to whip up anti-Russian sentiment.
"This story was straight away used to boost an anti-Russian campaign in the media," foreign ministry spokeswoman Maria Zakharova told reporters. "It is difficult to see (in the story) anything other than provocations aimed at harming the relations between our two countries."