Police have identified more than 240 witnesses and 200 pieces of evidence as they investigate the poisoning of Russian ex-spy Sergei Skripal and his daughter, the Home Secretary has said.

Amber Rudd revealed the scale of the probe as investigations into the attempted murder continued at a number of sites across the city centre.

A Zizzi restaurant thought to have been visited by the pair shortly before they became ill remain closed and cordoned off, amid unconfirmed reports that nerve agent traces were identified there.

Double agent Mr Skripal and his daughter remain in a critical condition.

Detective Sergeant Nick Bailey, who fell seriously ill after tending to the pair, yesterday said "does not consider himself a hero" and was "merely doing his job" in a statement released from hospital.

Cordons remain in place at a number of locations across the city, including Mr Skripal's house and the cemetery where his wife and son are buried.

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.

Specialist armed forces teams are also helping investigate and also clear scenes feared to have been contaminated with nerve agent.

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

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."

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.

Mr Bailey, who is still seriously ill, thanks supports for their messages to him as he recovers.

"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."