LONDON, March 9. /TASS/. The investigation into poisoning of former Russian military intelligence (GRU) Colonel Sergei Skripal and his daughter Yulia mirrors the case of former FSB officer Alexander Litvinenko who was poisoned in London in 2006, Russia’s embassy to the UK wrote on its Twitter page on Friday.

"Investigation of Sergei Skripal case follows the Litvinenko script: most info to be classified, Russia to get no access to investigation files and no opportunity to assess its credibility," the tweet says.

In 2004, Russia’s Federal Security Service (FSB) arrested Skripal and later on, he was sentenced to 13 years in prison for high treason. Six years later, the former colonel was handed over to the US as part of a swap involving espionage suspects. In the same year, Skripal arrived in the UK and settled there.

On March 4, the local police said the 66-year-old man and his 33-year-old daughter Yulia were found unconscious on a bench near The Maltings shopping center in Salisbury, following exposure to an unknown nerve agent. Both remain in hospital in critical condition.

Alexander Litvinenko was an officer with the Russian Federal Security Service, but he fled to the UK where he was granted refuge. On 23 November 2006, Litvinenko died in London. Experts say he was killed by radioactive polonium-210 but the circumstances behind his death have not yet been established and remain a matter for dispute. Lawyers of Litvinenko’s widow admitted that he had been working for the intelligence agencies of the UK (MI6) and of Spain before his death. He was granted a British passport shortly before his death.

The Financial Times said on Friday citing security officials that Skripal continued to provide information to western intelligence agencies after arriving in the UK in 2010. According to one former senior security official, Skripal was still valuable for "friendly’ spying agencies."

Earlier on Friday, Russian Foreign Minister Sergei Lavrov said that Russia was ready to assist in investigating any case, including the Salisbury incident if professional channels were used for request.