Russia as demanded that London provide proof that British spies did not carry out the poisoning of former double agent Sergei Skripal.

The Russian Foreign Ministry said in a statement that their analysis of the assassination attempt has them to believe in 'a possible involvement in it of the British intelligence services'.

The Ministry says that in the absence of proof of British innocence, Moscow will regard the incident as an attempt on the lives of Russian citizens on foreign soil.

Victims: Sergei Skripal, 66, and his daughter Yulia, 33, pictured, remain in critical condition following being poisoned with the highly lethal nerve agent Novichok in Salisbury on March 4

'An analysis of all the circumstances ... leads us to think of the possible involvement in it (the poisoning) of the British intelligence services,' the foreign ministry said in a statement.

'If convincing evidence to the contrary is not presented to the Russian side we will consider that we are dealing with an attempt on the lives of our citizens as a result of a massive political provocation.'

More than two dozen nations, including the U.S. and several EU nations, have expelled Russian diplomats in response to the nerve agent attack on Mr Skripal and his daughter Yulia in Salisbury three weeks ago.

Yesterday, NATO expelled seven Russian staff and denied accreditation to three more, bringing the total number of suspected Russian spies expelled to almost 150, including the 23 initially dispatched by Britain.

Demanding proof: Russia's President Vladimir Putin is seen speaking at a meeting on economic issues on Wednesday

The Kremlin said earlier today that a response to the recent expulsions of Russian diplomats from Western nations following the Skripal incident will come 'soon'.

Asked if Russia is going to react to the expulsions, Kremlin spokesman Dmitry Peskov told reporters on Wednesday that Russia's response will follow an that that it will be 'timely and will suit the interests of Russia.'

Skripal, a Russian military intelligence officer imprisoned by Moscow for passing on information about Russian agents in various European countries, came to Britain in a 2010 spy swap.