Theresa May is to urge EU leaders to stand together in the face of a threat from Russia which can be expected to last years into the future.
With UK-Russian relations plunged into the deep freeze by the nerve agent poisoning in Salisbury, the Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.
But she will insist that if European states stand shoulder to shoulder in the face of Russian aggression, then "united, we will succeed".
Her comments come a day after Foreign Secretary Boris Johnson escalated the war of words with Moscow by suggesting that Vladimir Putin is hoping for a propaganda boost from this summer's World Cup similar to that which Hitler sought in the 1936 Berlin Olympics.
As the war of words continued between Moscow and London, the Russian foreign ministry described Mr Johnson as "poisoned with hate and unprofessionalism."
Addressing fellow EU leaders at the European Council summit in Brussels, Mrs May will express her appreciation of the solidarity shown by allies across the world who have backed the UK's assessment that Russia is to blame for the attempted murder of Sergei and Yulia Skripal by nerve agent in Salisbury.
And she will warn that the "indiscriminate and reckless" attack fits in with a pattern of disrespect for international rules and norms by Moscow which threatens the basis for Europe's advanced democracies, open societies and free economies.
Although the attempted assassination of the Skripals took place, like the murder of Alexander Litvinenko, in the UK, it is clear that the Russian threat does not respect borders and places all European nations in risk, she will say.
"The challenge of Russia is one that will endure for years to come," Mrs May is expected to say.
"As a European democracy, the UK will stand shoulder to shoulder with the EU and with Nato to face these threats together. United, we will succeed."
Mrs May will welcome the joint statement issued on Monday by the EU's 28 foreign ministers, who voiced "unqualified solidarity" with the UK and support for its efforts to bring those responsible for the Salisbury attack to justice.
But Downing Street declined to say whether she would raise concerns over Jean-Claude Juncker's controversial letter congratulating Mr Putin on his re-election as president, in which the European Commission chief argued for "positive relations" between the EU and Russia.
In a mark of the decisive shift in Britain's view in the wake of incidents ranging from the Litvinenko and Skripal poisonings to military interventions in Georgia and Ukraine and cyber interference with Western elections, one senior Whitehall official said Russia had shown itself to be "a strategic enemy, not a strategic partner".
However the official stressed that Britain's response to the Salisbury attack had been carefully calibrated to remain within the law, and that the UK "is not looking for confrontation or regime change".
Mrs May will also voice her hopes that the EU27 will give its seal of approval on Friday to the draft agreement on the UK's transition to Brexit thrashed out by David Davis and Michel Barnier earlier this week.
The PM will leave Brussels on Thursday night. ahead of the EU27 talks which are expected to clear the way for post-Brexit trade negotiations to begin in earnest.
European Council president Donald Tusk raised doubts on Tuesday over whether agreement would be reached, with Spain understood to be insisting on the inclusion of a veto for Madrid on the future of Gibraltar and differences remaining over the treatment of the Irish border.
But on Wednesday, Mr Tusk was sounding a more positive note, saying he was "absolutely sure" that a solution could be found to avoid the risk of a hard border between Northern Ireland and the Republic.