As Tuesday night's deadline approached, nobody in Westminster expected Russia to offer any explanation of last week's attack in Salisbury that would stop Theresa May from taking punitive action in response.
When the National Security Council (NSC) considers Britain's options on Wednesday morning, it will seek to identify actions which are effective, sustainable and likely to win international support.
Housing minister Dominic Raab on Tuesday dismissed reports that the government was considering invoking Nato's Article 5, which obliges allies to come to the aid of any member state which is attacked. He pointed out that May had described the Salisbury incident as an "unlawful use of force" rather than an armed attack, adding that the range of measures could include "diplomatic measures, financial measures, economic measures and issues around visa bans and things like that".
Diplomatic measures could mean the expulsion of Russian spies operating in Britain under diplomatic cover, the expulsion of the Russian ambassador or the withdrawal of Britain's ambassador from Moscow. In 1971, at the height of the cold war, Ted Heath expelled 90 Russian spies operating under diplomatic cover, almost a fifth of the total number of Soviet diplomats in the country. Such a mass expulsion would not only be an eye-catching gesture but would damage the Russian intelligence services' capacity for action in Britain.
Freezing assets
May is likely to offer the government’s support for a version of the US Magnitsky Act, which targets individuals implicated in human rights abuses and large-scale corruption in Russia. Such a measure would allow Britain to freeze the assets of rich Russians close to the Kremlin and ban them from entering Britain.
The prime minister has spent much of the past 24 hours seeking the support of allies in Europe and elsewhere for co-ordinated action against Moscow. Angela Merkel and Emmanuel Macron expressed solidarity with Britain but have not yet taken up a call by the European Parliament's Guy Verhofstadt for EU leaders to agree joint counter-measures against Russia.
The EU is divided in its approach to Russia, with some right-wing parties in European governments enjoying close relationships with the Kremlin while other governments are reluctant to impose fresh sanctions that could hurt European businesses.
US president Donald Trump's sacking of Rex Tillerson as secretary of state appears to have been unrelated to Tillerson's strong expression of support for Britain. But the president's expression of solidarity with May fell far short of a commitment to join in any retaliatory action against Moscow.