UK Prime Minister Theresa May is braced to take “extensive measures” against Russia should it not offer a credible explanation of how an ex-spy and his daughter were poisoned on British soil with a military-grade nerve agent.
“Should there be no credible response,” Mrs May told parliament, “we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom”.
But what could the UK actually do – both on its own, and with the help of allies? And how likely are the US, EU and others to be on board?
Britain could expel Russian diplomats, as it did after the poisoning of former Russian Federal Security Service operative Alexander Litvinenko in 2006 with radioactive polonium.
But many argue that this, and the other measures that were taken after that killing – including visa restrictions on Russian officials – did not go far enough. The man identified as the main suspect, Andrei Lugovoi, is not just at large, he is now a Russian MP.
So what else could the UK do?
Current sanctions on Russia that Britain supports are imposed via the European Union. They were first passed after Russia annexed Ukraine’s Crimean peninsula in 2014 and backed rebels fighting in eastern Ukraine. Some 150 individuals and 38 companies have been targeted with visa bans and asset freezes.
EU countries are already divided on the sanctions, with diverging views among members states as to how Russia should be treated. States like Hungary, Italy and Greece have all supported the weakening of sanctions.
Some doubt whether Britain could convince the bloc to further toughen its measures against Moscow, especially with the UK on its way out of the Union.
By framing the poisoning as a possible “unlawful use of force” by Russia against the UK, Theresa May prompted questions as to whether this could be a matter for Nato, the military alliance of 29 countries.
The alliance’s policy of collective defence – under Article 5 – states that an attack on any one ally is seen as an attack on all.
It was invoked for the first and only time by the United States after the 9/11 attacks in New York.
Lord Ricketts, a former UK national security adviser, told the BBC that such an “unlawful act” warranted the involvement of Nato.
Any action “will be much more effective if there can be a broader, Nato-EU solidarity behind us”, he said.
But Downing Street has played down suggestions that this is an Article 5 matter.
For its part, Nato has called the attack “horrendous and completely unacceptable”. Secretary General Jens Stoltenberg said that the incident was of “great concern” to the alliance, which has moved in recent years to deter Russia by sending troops to Poland and the three Baltic states.
Lord Ricketts suggested one option involving Nato could be a reinforcement of resources on the group’s eastern flank.
The UK could also seek to bring the issue to the UN – and seek to gather international support for action against Russia.
Theresa May has already spoken to France’s President Macron and the two leaders “agreed that it would be important to continue to act in concert with allies”, according to Downing Street. Although Mrs May has not yet spoken to President Trump about the case – there have been “conversations at a senior official level”.
The UK has already internationalised the matter by asking Russia to provide a “full and complete disclosure” of the Novichok nerve agent programme to an international agency, the Organisation for the Prohibition of Chemical Weapons.
Indeed, the magnitude of the response that may be announced on Wednesday will depend on the scale of international co-operation that Mrs May can secure, says BBC Diplomatic Correspondent James Landale.
White House spokeswoman Sarah Sanders called the attack an “outrage” and Secretary of State Rex Tillerson went further, saying the attack “clearly came from Russia”. President Donald Trump himself has not spoken out.