(Adds details from Defra)
LONDON, April 17 (Reuters) - The nerve agent used to poison former Russian spy Sergei Skripal and his daughter at his home in southern England last month was delivered in a liquid form, British officials said on Tuesday.
Skripal, 66, who as a colonel in Russian military intelligence betrayed dozens of agents to Britain's foreign spy service, was found slumped unconscious on a bench in the city of Salisbury along with his daughter Yulia on March 4.
Britain has blamed Russia for the attack which the authorities said was carried out using a Novichok form of nerve agent. Moscow denies the accusation and says Britain is trying to whip up anti-Russian hysteria.
In a briefing to media on Tuesday, the Department for Environment, Food and Rural Affairs (Defra) which is handling a clean-up operation in Salisbury said a very small amount of Novichok was used with the substance delivered in a liquid form.
Police have said they believe the poison had been applied to the front door of Skripal's modest home as that was where the highest concentration of the toxin had been detected.
After weeks of treatment, Yulia Skripal, 33, was discharged from hospital a week ago. Her father remains in hospital, although he is no longer in a critical condition, and both are still receiving treatment for the effects of the poisoning.
Defra said nine sites, including three in Salisbury city centre, needed some level of specialist cleaning but some areas which had been sealed off since the poisoning were being reopened to the public.
"Thanks to detailed information gathered during the police's investigation, and our scientific understanding of how the agent works and is spread, we have been able to categorise the likely level of contamination at each site and are drawing up tailored plans," said Defra's Chief Scientific Adviser Ian Boyd.
The Organisation for the Prohibition of Chemical Weapons (OPCW) has confirmed Britain's analysis that a highly pure type of Novichok, developed by the Soviet Union in the 1970s and 80s, had been used in the attack on the Skripals. (Reporting by Michael Holden; editing by Stephen Addison)
Our Standards: The Thomson Reuters Trust Principles.