Russia was spying on former double agent Sergei Skripal at least five years before attempting to assassinate him with a military-grade nerve agent, British intelligence says.

The claims were detailed in a publicly-released letter from United Kingdom national security adviser Sir Mark Sedwill to NATO’s Secretary-General.

Sir Mark also alleged Russian agents had practised assassinating victims by putting the nerve agent Novichok on doorhandles — the method Britain says was used against the Skripals.

“During the 2000s Russia commenced a program to test means of delivering chemical warfare agents to train personnel from special units in the use of these weapons,” the letter said.

“The program subsequently include investigation of ways of delivering agents, including by application to door handles.”

Sir Mark’s letter also claimed Russian President Vladimir Putin was closely involved in Russia’s chemical weapons program in the mid-2000s.

The Skripals were found unconscious on a bench in the English cathedral city of Salisbury on March 4.

British Prime Minister Theresa May said the pair were attacked with a military-grade nerve agent from the Novichok group of poisons, developed by the Soviet Union.

The highest concentration of Novichok was found on the door handle at the home of Mr Skripal, whose emails Britain claims were targeted going back to 2013.

Russia continues to deny any involvement in the poisoning of the former double agent and his daughter.

Speaking at a press conference in London, Russia’s ambassador to the UK, Alexander Yakovenko, accused Britain of hiding the Skripals and hiding the truth.

“We get the impression that the British Government is deliberately pursuing the policy of destroying all possible evidence, classifying all remaining materials in making an independent and transparent investigation impossible,” Mr Yakovenko said.

-ABC