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?
Expel senior diplomats, perhaps even the Russian ambassador, and known Russian intelligence agents
Take some sort of action to bar wealthy Russian oligarchs from accessing their mansions and other luxuries in London, as suggested by Tory MP and House of Commons foreign affairs committee chair Tom Tugendhat. One way this could happen is through the use of Unexplained Wealth Orders, which allow government officials to seize assets including property until they have been properly accounted for.
A boycott of the Fifa World Cup in Russia later this year by officials and dignitaries - a symbolic move that UK allies are unlikely to emulate.
Taking Russian broadcasters such as RT (formerly Russia Today) off the air - broadcasting regulator Ofcom has said it will "consider the implications for RT's broadcast licences" after Mrs May speaks on Wednesday.
Pass a British version of the 2012 US Magnitsky act, which punishes Russians involved in corruption and human rights violations with asset freezes and travel bans. It is named after a Russian lawyer who died in custody after revealing alleged fraud by state officials. MPs have been pushing for a Magnitsky amendment to be added to the Sanctions and Anti-Money Laundering Bill now going through Parliament.