Russian President Vladimir Putin on Wednesday branded former double agent Sergei Skripal a "traitor" and a "scumbag", saying Moscow knew he had cooperated with foreign intelligence after his release in a spy swap.

But he again denied any Russian involvement in the poisoning of Skripal, who Britain says was attacked with a Soviet-designed nerve agent by members of Russia's military intelligence in March.

"He is just a scumbag," a visibly angry Putin told an energy forum in Moscow.

"He is just a spy, a traitor to the homeland," Putin said in his toughest remarks about Skripal to date.

"He was caught, he was punished, he spent five years in prison, we let him go, he left and continued cooperating with, providing consultations to (foreign) security services."

Skripal, a former Russian military intelligence colonel, was found guilty of passing state secrets to Britain and sentenced to 13 years in prison in 2006. He was pardoned by then-president Dmitry Medvedev and released as part of a spy swap with the West in 2010.

The Russian president once again laughed off British charges that two members of Russia's GRU military intelligence service had sought to poison Skripal and his daughter Yulia with Novichok in the English city of Salisbury in March.

Putin dismissed the crisis in relations with Britain that led to the largest expulsion of Russian diplomats from the West since the Cold War as a "row between security services."

"As we know, spying, like prostitution, is one of the world's most important professions," Putin said, drawing applause from the audience.

He again denied charges that Russia was behind the poisoning, saying: "No one had to poison anyone there (in Britain)."

"Sometimes I look at what is happening around this case and am simply amazed," said Putin, adding that  "The sooner (the Skripal scandal) is over, the better."