Theresa May said she would tell her fellow EU leaders that „the Russia threat“ did not respect borders, as she arrived for a summit in Brussels.
The UK PM said the nerve agent attack in Salisbury was „part of a pattern of Russian aggression against Europe“.
She will stress the UK’s commitment to European security, after it leaves the bloc in March 2019.
European leaders are also due to decide whether or not to agree the terms for a 21-month Brexit transition period.
Mrs May, who will not be present when the other EU leaders discuss Brexit on Friday, will brief her counterparts on Thursday on the poisoning of former Russian spy Sergei Skripal and his daughter in Salisbury.
The European Council is expected to adopt conclusions condemning the attack, which the UK government has said the Russian state was culpable for – but which Russia denies.