Since 2014 the EU has imposed a series of economic sanctions and other measures against Russia, including asset freezes and travel restrictions, over its illegal occupation of Crimea and destabilisation of Ukraine.
The Prime Minister is hoping to convince the leaders of the 27 other EU members states to back the continuation of sanctions.
Today, at the first working session of the summit she is expected to tell her counterparts: “Russia and other actors seem to be trying to sow disunity, destabilise our democracies and test our resolve.
“We must adapt our current defences to the ‘new normal’ and take responsibility for protecting international norms and institutions.”
In response to the Salisbury nerve agent attack, Mrs May will call for action to ensure suspected spies expelled by the UK and its allies cannot simply relocate to other EU countries.
She will also urge the European countries to work together against cyber attacks backed by Russia and other states.
This month, the US has slapped sanctions on five Russian companies and three Russian individuals for allegedly aiding Russia’s main intelligence agency.
At the summit, a senior UK Government official claimed Theresa May will also urge EU leaders to demonstrate their “shared responsibility to tackle illicit financial flows from those often linked to hostile regimes”.
The Prime Minister appeal will come only a day after the UK and its allies defeated Russia in a fierce vote in The Hague in a bid to give chemical weapons inspectors new powers to apportion blame for chemical weapons attacks.
Representatives of the Organisation for the Prevention of Chemical Weapons (OPCW) voted 82-24 in favour of backing the motion which was brought forward by Britain’s Foreign Secretary Boris Johnson.
UK diplomats succeeded in gaining sufficient support from OPCW members to pass the motion to strengthen the mandate of the agency, despite strong opposition from Russia.
Russia, Iran and Syria all strongly opposed the motion at the conference, and used a series of delaying tactics to try and halt the vote, but they were ultimately unsuccessful.
Russia was accused by the UK Government of poisoning Sergei and Yulia Skripal in Salisbury using the nerve agent novichok in March.
The Assad regime has also been accused of attacking its civilians with chemical weapons, such as the chlorine gas attack in the city of Douma in which over 70 people were killed.
The OPCW concluded in both cases that chemical weapons were used, but were unable to apportion blame to specific states.
However the Russian embassy in the Netherlands rejected these accusations, stating: “Deception is perhaps the word of a day.
“Has the UK presented any hard evidence for the so-called Skripals case? No.
“They embroiled their allies in the blatant campaign against Russia.”