Russia on Friday opened a probe into the "attempted premeditated murder" of Yulia Skripal, who along with her father was poisoned by a nerve agent in Britain.

The Investigative Committee has also begun a separate probe into the suspected murder of a Russian living in London, Nikolai Glushkov, even though British police were still treating the crime as "unexplained."

The committee, which reports directly to President Vladimir Putin, opened a case based "on the fact of attempted premeditated murder of Russian national Yulia Skripal," it said in a statement.

The statement did not mention Yulia Skripal's father Sergei, who is also in a critical condition in hospital after being poisoned with a Soviet-designed nerve agent called Novichok.

Russian investigators said that they were "ready for joint work with the relevant British authorities."

The Investigative Committee said in the same statement that it had opened a murder probe into the death of Glushkov, a former deputy general director at the Aeroflot airline and associate of the late Kremlin opponent Boris Berezovsky.

Glushkov was found dead at his home in London on March 12, days after the poisoning of the Skripals.

London's Metropolitan Police have so far been treating the death as "unexplained".

A spokesman told AFP Friday that the police were awaiting the results of a post-mortem expected later in the day.

The Russian embassy in London said this week that it had asked British authorities for details about Glushkov's death in the London suburb of New Malden.

Glushkov was an associate of Berezovsky, a one-time Putin supporter who then turned against him, and was found hanged in a bathroom at his home outside London in 2013.

Glushkov had received political asylum in Britain after being held in pre-trial detention and convicted in Russia for money laundering and fraud.

British police are revisiting a number of other unexplained deaths following the Skripal case.