A dose of the nerve agent hunted by police in the Salisbury and Amesbury areas could be active for up to 50 years, police have said.

The Metropolitan Police's head of counter-terror, Neil Basu, made the comments at a public meeting in Amesbury.

He was asked if the novichok could be in a landfill site.

In response, Assistant Commissioner Basu said: "If it is sealed in a container and it was in a landfill site it would effectively be safe because it would not be touched by anyone and it would last, probably, I've been told by scientists, 50 years."

He was then asked if he was looking for a "needle in a haystack" and he said: "That's why we need witnesses or intelligence."


Image: Amesbury, where Dawn Sturgess and Charlie Rowley were exposed to novichok

The nerve agent was used to target former Russian spy Sergei Skripal and his daughter Yulia in Salisbury in March. The two of them spent many weeks in hospital but survived the attack.

It also killed 44-year-old British woman Dawn Sturgess in Amesbury and left her partner Charlie Rowley in hospital, where he has recently made a "small but significant" improvement.

It is thought that Mr Rowley and Ms Sturgess were exposed to novichok 10 days ago through a contaminated item left behind after the attack on the Skripals.

How does novichok affect the body?

Police cannot yet say that the two incidents are definitely linked, Mr Basu said.

However, he added: "This is a very rare substance banned by the international community and for there to be two separate, distinct incidents in one small English county is implausible to say the least."

He said: "The brutal fact is we don't know where (Mr Rowley and Ms Sturgess) found it.

"I am hoping Charlie recovers and when he recovers he will be able to tell us and perhaps shed some light on it which will narrow our search dramatically.

"There is a possibility they found it on 5 March and only opened it in the past 10 days."

Image: Charlie Rowley and Dawn Sturgess

Assistant Commissioner Basu told residents: "This is murder, you know it's murder, so I don't need to stiffen the resolve of the detectives who are working on this case to try and get justice for Dawn, her family and everyone who has been affected, including this community.

"I would love to be able to stand and say to you that we've identified and caught the people responsible, how we're certain there are no traces of nerve agent left anywhere in your county, but the brutal reality is I cannot offer you any such assurance or guarantee at this time."