Moscow has rejected a British ultimatum to explain why a Soviet-developed toxin was allegedly used to poison a former Russian spy on British soil.

It said it will ignore the ultimatum until the UK hands over samples of the alleged nerve agent used in the attempted murder of Sergei Skripal and his daughter Yulia, something Moscow says is an obligation under the Chemical Weapons Convention.

"Before issuing ultimatums to report before the British government within 24 hours it is better to follow your own obligations in accordance with the international law, in this case with the Chemical Weapons Convention. And in general, speaking about manners, it's good to remember that the era of colonialism is a thing of the past," Russian Foreign Minister Lavrov said.

Sergei and Yulia remain in a critical condition in hospital after being poisoned with a nerve agent in the English city of Salisbury.

The British government has identified the poison as part of the Novichok group of nerve agents developed by the Soviet military during the 1970s and 1980s.

Prime Minister Theresa May said it is "highly likely" Russia is to blame.

The Russian ambassador has been summoned to provide an explanation. Without one, May has warned of "extensive" measures, though what that means is not clear.

If Britain were to impose sanctions it could do with some support from the international community.

The EU has expressed solidarity with the UK, but diplomats told Reuters there is little appetite for more economic sanctions on Moscow.

Meanwhile, US President Donald Trump said the United States still needs to sort out the facts around the attack, but added that he would condemn Russia if they support the UK's position.