The Russian Investigative Committee said Friday that it has opened a criminal investigation into the attempted murder of Yulia Skripal, daughter of a former Russian spy poisoned in Britain.
"The investigation will be carried out in accordance with the requirements of Russian and international law," the committee said in a statement.
It said the investigators were ready to work together with competent British authorities.
Former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench outside a shopping center in Britain's southwestern city of Salisbury on March 4. They remain in critical condition in hospital.
Britain claims the pair was exposed to a nerve agent and holds Russia responsible. Moscow has denied any involvement.
Britain on Wednesday announced the expulsion of 23 Russian diplomats, freezing of Russian state assets in Britain, suspension of all planned high-level bilateral contacts, and a boycott of this year's World Cup in Russia by ministers and royal family members.
Russian officials said Moscow would retaliate, but no measures have been announced so far.
The Russian Investigative Committee said it has also opened a criminal case over the suspected murder of Nikolai Glushkov, a Russian exile and a close friend of late Russian oligarch Boris Berezovsky. Glushkov was found dead in his London home on Monday.