Russia has asked for access to samples of the nerve agent that poisoned ex-spy Sergei Skripal and his daughter Yulia in Salisbury, Foreign Minister Sergei Lavrov has said.
Mr Lavrov said Russia “is not to blame” for the incident on March 4.
The Russian Foreign Ministry has summoned the British ambassador in Moscow over the case, Russian news agencies have said.
Mr Lavrov warned that Russia will only co-operate with Britain on the investigation if it receives samples of the nerve agent that is believed to have been used to target Mr Skripal and his daughter.
Mr Lavrov said that Moscow’s requests to see samples of the nerve agent have been turned down, which he called a violation of the Chemical Weapons Convention, which outlaws the production of chemical weapons.
Prime Minister Theresa May said on Monday that Mr Skripal and his daughter had been poisoned with a military-grade nerve agent of a type developed in the Soviet Union.
She said Russia has until the end of Tuesday to explain how the substance ended up in Britain.
Mr Lavrov said on Tuesday that Moscow is willing to co-operate with the probe, but he suggested London would be “better off” complying with its international obligations “before putting forward ultimatums”.