The nerve agent used to poison former spy Sergei Skripal and his daughter Yulia was delivered "in a liquid form", the Department for Environment says.

The pair were discovered unconscious on a bench in Salisbury on 4 March.

A massive clean-up operation is beginning to decontaminate nine sites in the city - it will take several months and cost millions of pounds.

The UK government says Russia was behind the poisoning, but Moscow denies any involvement.

Yulia, 33, left hospital earlier this month. Her 66-year-old father is said to be recovering more slowly.

The Department for Environment, Food and Rural Affairs (Defra) said a "very small amount" of the military grade Novichok nerve agent was used in the attack, with the substance delivered in a "liquid form".

It will be cleaned away with caustic agents and hoardings will go up in public places in the next few days for the clean-up to begin.

The nine sites affected include ambulance stations, Sergei Skripal's house, and various city centre locations, including Zizzi restaurant and the Mill pub, where the Skripals spent time on the day they fell ill.

Grass and stones will be removed from public areas around the Maltings shopping centre. All material taken away will be incinerated.

Nearly 200 military personnel will help with the work, which is expected to take a number of months.

A tenth site - a section of London Road cemetery - has now been reopened to the public after tests established it was not contaminated.

Watchdog findings
Defra's chief scientific adviser Ian Boyd said its approach was "based on the best scientific evidence and advice to ensure decontamination is carried out in a thorough and careful way".

"Our number one priority is making these sites safe for the public, so they can be returned to use for the people of Salisbury."

The Skripals' poisoning has triggered a diplomatic crisis between Russia and the West, with more than 20 countries expelling Russian envoys in solidarity with the UK.

The international watchdog, the Organisation for the Prohibition of Chemical Weapons, has backed the UK's findings regarding the substance responsible.

The UK has called for a UN Security Council meeting on the OPCW report, which could be held this week.