Yulia Skripal is also in a critical condition.
Malcolm Sperrin, the former head of medical physics at the Royal Berkshire Hospital, said it would normally take time for a radioactive dose to become evident.
She gained a master's in economics from the People's Friendship University of Russia before coming to visit London in 2001 and meeting her public schoolboy Alex Chapman at an underground rave in the Docklands area. Police declared a major incident.
It is thought her latest visit to the UK may have been timed to coincide with her late brother's birthday. There is a protocol between the two intelligence agencies, a sort of honour among thieves. "We can only see it as a provocation".
The 66-year-old appears relaxed in the shop on the corner of Roman Road and Wilton Road.
While the network's main role is to prevent and investigate terrorist activity, it can also be assigned to other specialist probes including inquiries relating to war crimes, counter-espionage and the Official Secrets Act.
The gravestone of Liudmila Skripal, the wife of former Russian double agent Sergei Skirpal.
"I was surprised it should happen in a little city like Salisbury - London perhaps you expect it but not so much in somewhere like this city because it's quite rural".
Police have declined to specify the nerve agent used, which was identified in scientific tests by Government experts.
"There is a lot of information about him but I am not going to comment further about that", she said. "But this does certainly bear all the hallmarks of what the Russians call 'wet work, ' or an assassination attempt".
Bernice Marsh, 52, manager of Maltings business Party Seasons, added it was a "shock" to hear about Mr Skripal and his daughter.
The case has been compared to the 2006 murder of the former Russian intelligence officer Alexander Litvinenko.
In another incident, Pavel Sheremet, a journalist (and Putin critic), was killed when the car he was driving exploded.
And one of them was this man Sergei Skripal who then came to the U.K. where he was living very quietly until just the last few days.
He warned Britain would respond "appropriately and robustly" if a government was found responsible.
Before the basic facts of the case have been established, both sides have indulged in an early exchange of fire.
Britain has seen cases of Russians who have criticized the Kremlin meet dubious ends here in the past decade. What evidence is there that Russia may have played a role, that there was some foul play happening here?
Sergei (left) and Yulia Skripal were found unconscious outside London.
"Readers are presented with various theories, which boil down to ways of demonising Russia".
Russian Foreign Ministry spokeswoman, Maria Zakharova, said: "These people have been used by the foreign media for an anti-Russian campaign".
"Those who serve us with poison will eventually swallow it and poison themselves", he said in an interview with Russian state television.
Once a colonel in Russia's GRU military intelligence service, he was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style swap in Vienna.