Assistant Commissioner Mark Rowley said: "We are now treating this as a major incident involving an attempted murder by the administration of a nerve agent.

"We believe the two people who originally became unwell were targeted specifically."

Hundreds of police officers are working on the case, examining thousands of hours of CCTV footage, and they will remain in Salisbury for several days, Mr Rowley added.

A former Russian spy and his daughter were targeted with a nerve agent in Salisbury, police have revealed.

Detectives are investigating the attempted murder of Sergei Skripal and his daughter Yulia, who are fighting for their lives in hospital.

A Wiltshire Police officer, who was one of the first at the scene, is also in a serious condition and understood to be in intensive care.

The Metropolitan Police, which is leading the investigation, said there was no evidence of a widespread health risk.

---

'The UK will respond robustly'

Foreign Secretary Boris Johnson has described the events in Salisbury as "very troubling".

He said: "If this does turn out to be in any way the result of hostile activity by another government, or directed, led, by another government, then the people of this country can be absolutely sure that the UK will respond robustly."

What is a nerve agent?
A nerve agent works by interrupting the messaging from the brain down into the muscles.

Sky News Science and Medical Correspondent Thomas Moore says: "In essence the nerves fire too much too quickly, they become tired and the muscles stop working."

What are the symptoms?
The victims of this suspected poisoning had difficulty breathing, and were described by witnesses as "wheezing".

They were discovered unconscious on a park bench in Salisbury Maltings shopping centre vomiting, twitching and "slumped" together.

The victims remain in a critical condition in quarantine in hospital

---

:: VX nerve agent
VX is an extremely toxic chemical compound that was originally developed for military use.

Just 10 milligrams of VX, when absorbed through the skin, is enough to cause "fatal disruption of the nervous system" within 15 minutes.

It is a clear, colourless liquid which is hard to detect.

VX penetrates the skin and disrupts the transmission of nerve impulses. Those exposed to the agent experience convulsions, loss of consciousness, paralysis and fatal respiratory failure.

It is believed to have been the substance used to assassinate Kim Jong Un's half-brother Kim Jong Nam at the airport in Kuala Lumpur, Malaysia, on 13 February last year.