Parents in the towns at the centre of nerve agent poisonings have been warned not to let their children pick up “any foreign object which could contain liquid or gel”.

Chief Medical Officer Dame Sally Davies stressed that no-one in Salisbury and Amesbury, particularly youngseters as the school holidays are starting, should pick up anything that they have not dropped themselves.

She said: “I want to emphasise to everyone in the Salisbury and Amesbury area that nobody, adult or child, should pick up any foreign object which could contain liquid or gel, in the interests of their own safety.

“This, in practice, means do not pick up containers, syringes, needles, cosmetics or similar objects, made of materials such as metal, plastic or glass.

“This is particularly important as families are starting to prepare for their children’s summer holidays and so I am asking that people are extra vigilant.

“To be clear: do not pick up anything that you haven’t dropped yourself.”

The advice from Public Health England remains that the overall threat to the public is low, but that everyone should avoid picking up strange objects.

It is believed that Dawn Sturgess, 44, who died from Novichok poisoning, and her partner, Charlie Rowley, 45, who is critically ill, somehow handled a vessel that contained the deadly substance.

There is no evidence to suggest they went to any of the areas that had been decontaminated following the attempted murders of Sergei Skripal and his daughter Yulia, who were also exposed to the nerve agent.

Dame Sally added: “Life continues in Salisbury and Amesbury and I want to be clear that the overall threat to the public is unchanged and remains low.

“I also want to highlight that those areas that have been cleaned and released to the public over the past few weeks are safe, and should be used normally.

“You do not need to seek advice from a health professional unless you are experiencing symptoms.

“If you are concerned, you should call NHS 111 and Wilshire Police has also established a helpline to offer further advice: 0800 092 0410.

“As before, you should continue to follow the advice of the police and that of Public Health England.”