Mr Skripal and his daughter Yulia were targeted with the nerve agent Novichok on 4 March 2018 in Salisbury.

Sgt Tracey Holloway said after finding his name online they realised "it could be something bigger".

A church service, ahead of the first anniversary of the attack, it due to take place later.

St Thomas's Church, in the city, will remember the victims and offer thanks to the community.

Mr Skripal's house and 11 other potentially infected sites were ruled safe on Friday.

The operation included taking thousands of test samples from across Salisbury and nearby Amesbury, where Dawn Sturgess, 44, was fatally poisoned in July.

Britain has accused Russia of carrying out the poisoning of the Skripals.

Russia has also been blamed for the death of Ms Sturgess and the poisoning of Mr Rowley, who are believed to have come into contact with a bottle of Novichok discarded by the Skripals' attackers.

Sgt Holloway said they knew "relatively early that they were both going to intensive care".

But police had initially thought the Skripals' collapse had been caused by a drugs overdose.

"The paramedics said they weren't sure what it was and we didn't know what they were suffering with," she said.

"They weren't dressed in the way I would expect a drug user to be, so I wasn't really sure what we had."

When the officers went back to the station they made the link to Russia - and the possibility of a much bigger case - as they found Mr Skripal's name online.

"It was actually another CID officer who had Googled his name and then said, 'Tracey, do you want to come and have a look at this?'

"It was at that point that we got the link to the Russians side of things. And at that point we thought this could be something bigger than what we believed could be a drugs overdose."

Sergei and Yulia Skripal survived the attack following treatment at Salisbury District Hospital.

Det Sgt Nick Bailey, who searched their house following the poisoning, returned to work last year and is preparing to run a marathon to raise money for the hospital.

More about: Russia   Sergei-Skripal   Salisbury   Google