The UK is sharing secret intelligence about the nerve agent attack on a former spy with key allies, in an effort to persuade them to expel Russian diplomats across Europe, people familiar with the matter said.
Prime Minister Theresa May ordered 23 Russian operatives – who she said were undeclared spies – to leave Britain last week in retaliation for the attack on Sergei and Yulia Skripal in Salisbury, England.
Now she wants other countries to follow Britain’s example, condemning Russia and taking action against Kremlin agents, according to the people, who asked not to be named discussing sensitive security matters.
Whether or not Mrs May succeeds is being seen as a key test of her ability to influence the EU and demonstrate that the UK can still count on its international friends after Brexit.
So far, however, EU governments have only issued a statement and have stopped short of joining Mrs May in blaming the Kremlin, with Greece said to be among the countries seeking further proof of Russian involvement.
"The Skripal affair is proving to be the first big foreign policy test for the UK after Brexit," said Mujtaba Rahman, a Europe analyst at Eurasia Group. "This week's mild statement by EU foreign ministers shows the limits of the UK's influence in the EU after Brexit."
Mrs May will deliver a hard-hitting warning over the threat posed to the whole of Europe by Vladimir Putin’s government during a dinner with fellow leaders at the EU summit in Brussels on Thursday, a senior British official told reporters.
“The challenge of Russia is one that will endure for years to come,” according to extracts released in advance by her office, Mrs May will tell the leaders.
The premier is hoping to persuade her EU counterparts to show solidarity by stepping up their criticism of Mr Putin and ensure that they lay the blame at Russia's door.
British officials say the world – including the EU – must show Russia that the first use of nerve agent on European soil since the end of World War II will not go unpunished. One option being discussed between the UK and its allies is a wide wave of expulsions of Russian diplomats from embassies across the West.
Russia denies that it is behind the nerve agent attack. The US, France, and Germany are among the countries to have publicly supported the UK's findings that Russia is culpable.
In particular, the UK is in constant contact with sceptical countries such as Greece in an attempt to persuade them of the evidence against Mr Putin, the people said.
"The Skripal case may also prompt remaining EU states to consider their preparedness for the threat posed by Russia," said Rahman in an email on Wednesday. "The main obstacle to a stronger reaction is the presence of Russia-friendly member states, not Brexit. This said, Brexit has made it harder for the UK to counter this opposition and push the EU27 to go the extra mile."
The UK says there is no doubt that the nerve agent used in the attack was of the type known as Novichok, which is manufactured in Russia. Officials have said it is likely Mr Putin himself ordered the attempted assassination of Mr Skripal, who had worked as a double agent for British intelligence.
Investigators in the UK now know exactly what type of nerve agent in the Novichok family was used, and precisely where in Russia it was made.
Mrs May has drawn up a package of measures in response. These include freezing Russian state assets "wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents." Police will also target "corrupt elites" because "there is no place for these people – or their money" in the UK, Mrs May told Parliament last week.
In a tit-for-tat move, Mr Putin expelled 23 British diplomats from Russia. Mrs May has not responded publicly to the act, but her officials have drawn up options for stepping up the pressure on the Kremlin.
Mrs May's office believes the removal of 23 Russian spies will help wind up Putin's espionage capability in the UK If British intelligence identifies other secret Russian agents, they will also be ejected from the country.
When she addresses the summit dinner, Mrs May will argue that Russia is a strategic enemy, not a strategic partner, saying it poses a long-term national security threat to all EU countries. The attack happened to take place in England but could have occurred in any European country, she will say, according to a UK official.
"As a European democracy, the United Kingdom will stand shoulder to shoulder with the European Union and with Nato to face these threats together," Mrs May will tell the leaders. "United we will succeed."