The nerve agent used to attack former Russian double agent Sergei Skripal and his daughter was delivered in a “liquid form”, the Department for Environment, Food and Rural Affairs (Defra) has said.

Only a “very small amount” of the deadly Novichok substance was used against Mr Skripal, 66, and 33-year-old Yulia, it is understood.

The highest concentration was found at Mr Skripal’s home on the outskirts of Salisbury, with eight other areas across the city potentially contaminated.

Further details about the substance emerged at a press briefing on Tuesday.

“In this instance, direct contact is required for a person to poisoned. Only a small proportion of the material is transferred in each contact and the substance is diluted in each secondary or tertiary contact. The class of nerve agent does not produce significant vapour or gas and can only be moved between sites by direct transfer from a contaminated person or by moving a contaminated item,” a spokesman for Defra said.

Asked what form the nerve agent was in, the Defra spokesman said: “It’s in a liquid form.”

It comes as the first of 10 sites cordoned off across the city was reopened to the public on Tuesday.

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