'Highly likely' Russia is behind ex-spy poisoning - Theresa May

British PM Theresa May says it's "highly likely" Russia is behind an ex-spy's poisoning in Britain last week.

Sergei Skripal and his daughter, Yulia Skripal, remain in a critical condition in hospital after being slipped a suspected nerve agent.

Nerve agent used on ex-spy found in restaurant - reports

Mr Skripal was once a colonel in Russia's GRU military intelligence service, but had been secretly working for the UK since the mid-1990s.

He was arrested by the Russians in 2004, but handed over to the UK in 2010 as part of a spy swap.

Ms May told British MPs in parliament on Monday (local time) if Russia is proven to be behind the poisoning, it will be considered an "unlawful use of force".

"It is now clear that Mr Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia.

"This is a part of a group of nerve agents known as novichok."

Russia's ambassador to the UK has been asked for an explanation.

Russia has denied responsibility.