London: British Prime Minister Theresa May is likely to announce a range of measures against Moscow today over the poisoning of a former Russian spy with a deadly nerve agent in England this month.

The British Prime Minister will chair a meeting of the UK's National Security Council (NSC) to lay out her proposals before announcing them in the House of Commons later today.

Moscow, which had been given an ultimatum to provide a "credible response" over how a Russian-made nerve agent came to be used on British soil, has warned of "an equal and opposite reaction" against any UK reprisals.

The set of measures are expected to cover the expulsion of Russian diplomats, financial sanctions against wealthy Russians with links to the Kremlin, possible curbs on the Russia's state-funded television station RT, and boycotting the FIFA World Cup in Russia later this year.

The reprisals will follow days of diplomacy since Theresa May first informed Parliament that there was enough evidence to conclude that it is "highly likely" that Russia is behind the poisoning of 66-year-old former Russian double agent Sergei Skripal and his 33-year-old daughter Yulia in Salisbury on March 4.

Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations; the government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal, she had said.

"It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk. And we will not tolerate such a brazen attempt to murder innocent civilians on our soil, she added, giving Moscow a Tuesday midnight deadline to respond on the circumstances surrounding the attack.

US President Donald Trump, as well as North Atlantic Treaty Organisation (NATO) and European Union (EU) allies including Germany's chancellor, Angela Merkel, have since come out in support of the UK and offered support.

Downing Street said the British prime minister received the backing of Trump, who agreed in a phone call that Moscow "must provide unambiguous answers as to how this nerve agent came to be used".

A spokesperson added that France's President Emmanuel Macron and German Chancellor Angela Merkel had also condemned the attack and offered support to the UK, as well as Estonia, Latvia and Lithuania the Baltic states bordering Russia.

Russia, however, has insisted that it is being blamed unfairly, with the Russian embassy in the UK tweeting that the ambassador, who had been summoned to the UK Foreign and Commonwealth Office (FCO) for an explanation, had branded the actions of the UK authorities as a clear provocation .

Russian Foreign Minister Sergei Lavrov accused Britain of "playing politics" and ignoring an international agreement on chemical weapons. He said Moscow would cooperate if it received a formal request for clarification from the UK under the Chemical Weapons Convention, which sets a 10-day time limit for a response.

UK foreign secretary Boris Johnson has said if the attack was shown to be a "direct act" by the Russian state it would be a "clear violation of the chemical weapons convention, a breach of international law and a threat to those who abide by the rules-based international order".

FCO is also set to brief a session of the North Atlantic Council, NATO's political decision-making body, on the Skripal incident later.

In her Commons statement earlier this week, May made a specific reference to the use of radiological substances in the "barbaric assault" on Alexander Litvenenko another Russian spy murdered 11 years ago in London.

Following Litvinenko's death, the UK had expelled Russian diplomats, suspended security cooperation, broke off bilateral plans on visas, froze the assets of the suspects and put them on international extradition lists.

May told Parliament that while those measures remain in place, "we must now stand ready to take much more extensive measures .

Meanwhile, Skripal, and his daughter remain critically ill in hospital. A former Russian military colonel, Skripal was convicted of treason in 2006 and jailed for 13 years for selling secrets to MI6, which had recruited him in the 1990s.

The senior intelligence officer with Russian military intelligence GRU, was pardoned in a spy swap in 2010 and settled in Salisbury, Wiltshire.

A week after he and his daughter were found collapsed on a bench of a shopping centre in the city, Britain' s public health authorities had asked hundreds of people who were at a restaurant and pub linked with the poisoning of the Russian spy to clean up their possessions to remove any traces of the deadly nerve agent.

The poison used in the attack has been identified as belonging to a group of chemical nerve agents known as Novichok', which means newcomer and was used by the Soviet Union during the Cold War era in 1970s and 1980s.

Like most nerve agents, it has the effect of blocking messages from the nerves to the muscles in the body, which leads to a collapse of body functions and ultimately death by asphyxiation.  PTI