It took several days for thallium poisoning to be confirmed

Their condition is described as "fairly serious". They were staying at a Moscow hotel and fell ill on 24 February, but the circumstances are not yet clear.

Russia's Ekho Moskvy radio said doctors had confirmed that the women, a mother and daughter aged 42 and 26, had been poisoned with highly toxic thallium.

The Soviet KGB used thallium against some of its enemies in the Cold War.

Thallium was initially suspected in the poisoning of former Federal Security Service agent Alexander Litvinenko, before it was realised that the toxin was polonium-210.