Mrs May warned EU leaders that Vladimir Putin’s Russia poses a long term threat to Europe and sought unity to face the threat of Russian aggression.
As Prime Minister left an EU council meeting, where she secured a joint statement blaming Russia for the Salisbury attack, she said: “The threat from Russia is one that respects no borders.
“I think it is clear that Russia is challenging the values we share as Europeans, and it is right that we are standing together in defence of those values.”
Asked whether she had provided EU allies with new intelligence material to secure their backing, Mrs May said: “We have been throughout sharing on intelligence channels what intelligence we can share with our colleagues.
“What is crucial is that there was recognition around the table last night about the threat that Russia poses.”
Yesterday the EU’s ambassador to Moscow has been temporarily withdrawn over the Salisbury nerve agent attack.
European Council President Donald Tusk revealed EU member states backed Britain’s claim there is “no other plausible explanation” than Moscow’s culpability.
Reports suggest as many as five EU countries including France, Poland and the three Baltic nations, could now follow Britain’s lead by expelling suspected Russian diplomats.
In her address to the other 27 EU leaders the Prime Minister set out evidence against Russia.
Mrs May provided information on the positive identification of the nerve agent Novichok by experts at Porton Down.
She also provided intelligence that Russia has produced this nerve agent within the last 10 years and remains capable of doing so.
Information was also provided on Russia’s record of conducting state-sponsored assassinations and that Britain holds the view that Russia sees defectors as suitable targets for assassination.
The Lithuania President Dalia Grybauskaite said that the meeting was “important” and that Mrs May had provided detailed and trust worthy information that influenced and united EU leaders to take action.
Speaking at the European Council in Brussels Miss Grybauskaite said: “I think the discussion was very deep, quite interesting and important.
“We got more detailed information from Prime Minister May and it was very good information, I mean trustable.”
President Grybauskaite revealed that she now better informed after the meeting and she expected several countries to take measures against Russia from the beginning of next week.
Asked if she had learnt more yesterday than she knew before Miss Grybauskaite said: “Of course and this came from Theresa May’s lips and it is important which information influenced our decision which, was unanimously to upgrade the text and to use the intelligence service formulation about the difference and relations with Russia.
“This allowed us to be very much united.”