Russian President Vladimir Putin has said it was "nonsense" to think that Moscow would have poisoned former Russian spy Sergei Skripal and his daughter, who are critically ill in a British hospital.

Britain has said that Mr Skripal and his daughter were poisoned by the Soviet-era 'Novichok' nerve agent, and British Foreign Secretary Boris Johnson said today that Russia has been stockpiling it and investigating how such weapons could be used in assassinations.

But Mr Putin, in his first detailed comments on the poisoning, has said Russia is falsely accused.

"As for the tragedy that you mentioned, I found out about it from the media," he said.

He added: "The first thing that entered my head was that if it had been a military-grade nerve agent, the people would have died on the spot.

"Secondly, Russia does not have such (nerve) agents."

He said: "We destroyed all our chemical weapons under the supervision of international organisations, and we did it first, unlike some of our partners who promised to do it, but unfortunately did not keep their promises".

On Monday, technical experts from the Organisation for the Prohibition of Chemical Weapons are to visit Britain to collect samples of the nerve agent used in the attack.

Despite the tensions, Mr Putin said Moscow was ready to cooperate with London.

"We are ready to cooperate, we said that straight away, we are ready to take part in the necessary investigations, but for that there needs be a desire from the other side, and we don't see that yet," he said.

"We are not taking it off the agenda, joint efforts are possible," he added.

"As a whole, of course, I think any sensible person would understand that it would be rubbish, drivel, nonsense, for Russia to embark on such an escapade on the eve of a presidential election."

"It's just unthinkable," he added.

Mr Putin was speaking after winning a new term in a presidential election.

British officials have said Moscow was culpable for the poisoning and expelled 23 Russian diplomats based in London.

Moscow retaliated on Saturday by expelling the same number of British diplomats from Moscow, shuttering the British consulate in Russia's second city of St Petersburg and closing down the Russian activities of the British Council, which promoted British culture overseas.