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Police have discovered traces of the nerve agent used against the former Russian spy Sergei Skripal and his daughter Yulia in the restaurant where they ate before they were found collapsed on a bench.

During an ongoing forensic examination, the substance was found at Zizzi in Salisbury.

The pair are currently in critical condition in hospital.

No-one else who was at the restaurant at the time is thought to be at risk, nor has it been suggested that their fellow diners had anything to do with the suspected attack, the BBC reported.

Zizzi is one of five sites that are currently being scoured for evidence by police.

Other areas that are under investigation are Mr Skripal’s home, the Mill pub which was visited by the Skripals, the bench where the father and daughter were found and the cemetery where Mr Skripal’s wife and son are buried.

Armed forces personnel were assisting the police with the probe for the third day on Sunday.

Soldiers were seen at the South Western Ambulance Service station on Saturday as a vehicle was winched on to the back of an Army low-loader and taken away.

It comes as Home Secretary Amber Rudd said police investigating the suspected nerve agent attack in Salisbury have identified more than 240 witnesses and 200 pieces of evidence.

There was further police activity at the London Road cemetery on Saturday, where officers in hazmat suits had removed items and covered his son's memorial stone with a forensic tent.

Scotland Yard said no exhumations had taken place.

Speaking following a meeting of the government's Cobra emergencies committee, Ms Rudd said there were more than 250 officers from eight out of 11 of the country's counter-terrorism units involved in the investigation.

She said: "I want to stress that they are proceeding with speed and professionalism. We are putting in enormous resources to ensure that they have all the support that they need to do that."

Ms Rudd said it was still too early to say who was responsible for the attack.

She said: "This investigation is focused on making sure that we keep people safe and also that we collect all the evidence so that when it comes to attribution (of the attack) we will be absolutely clear where it should be," she said.

"The police have said that if anybody thinks they have any additional information they would welcome them coming forward.

"There is also substantial amounts of CCTV they have to go through. This is a painstaking, detailed investigation and the police need to be given the space and time to get on with it."

Mr Skripal and his daughter Yulia are still fighting for their lives after being exposed to a toxic substance in the Wiltshire city.

Mr Bailey, who was part of the initial response by authorities, also remains in hospital, although he released a statement thanking people for their support.

The statement read: "Nick would like us to say on his behalf that he and his family are hugely grateful for all the messages of support from the public, and colleagues from the police family. People have been so kind and he has expressed that he will never forget that kindness.

"He also wishes to say that he was part of a group of officers and other emergency service colleagues who dealt with the initial incident.

"He wants to say that he does not consider himself a 'hero', he states he was merely doing his job - a job he loves and is immensely proud of - just like all of his other dedicated colleagues do, day in day out, in order to protect the public and keep people safe.

"He would like to thank everyone once again for all of their kind thoughts and best wishes, they are truly appreciated.

"He asks respectfully that the media allow his family privacy at this difficult time."

Police said 21 people had been seen for medical treatment since the incident.

The figure includes members of the public and emergency staff, some of whom have had blood tests as well as receiving support and advice.

The attack is being treated as attempted murder.

Additional reporting by Press Association