Theresa May will seek to rally European Union leaders to maintain a strong line against Russia at a summit in Brussels.
The Prime Minister will also update EU counterparts on Brexit, but the response to the migrant crisis is set to overshadow other topics during the meeting.
On Russia, Mrs May hopes the leaders of the 28 EU countries will back the continuation of sanctions imposed following the annexation of Crimea.
But she will also stress the need for further action, including measures to tackle propaganda and disinformation spread by Moscow.
She will also call for action to make sure suspected spies expelled by the UK and its allies in response to the Salisbury nerve agent attack cannot simply relocate to other EU countries.
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 an effort to tackle cyber attacks backed by Russia and other states, she will call on EU countries to work together to attribute blame and co-ordinate responses.
She will also urge EU leaders to demonstrate their "shared responsibility to tackle illicit financial flows from those often linked to hostile regimes", a senior UK Government official said.
The PM will provide an update on the state of the Brexit negotiations over dinner on Thursday evening, with the remaining 27 leaders set to consider progress in a separate session on Friday - after Mrs May has left the summit.
The political significance of the EU's response to the migrant crisis has escalated in recent weeks, with German Chancellor Angela Merkel facing major domestic difficulties over the issue.
Mrs May will restate the UK's commitment to working with the EU to "address the common challenge posed by illegal migration now and after the UK leaves the EU", an official said.
The Prime Minister believes that although tackling the current crisis in the Mediterranean is a priority, efforts must also be made to address the problem at its root.
She is expected to update EU leaders on a project funded by UK aid cash in Nigeria which is aimed at raising awareness of the dangers faced by people risking the journey to Europe.
The information campaign warned that migrants could end up in situations of modern slavery and forced labour in Europe and feedback showed that 97% of the 10,000 people reached by the scheme changed their attitudes to illegal working in the UK.
Some 78% said they no longer thought it was worth paying smugglers or risking their lives at sea to travel to Europe.
But schemes such as that are unlikely to address the pressures felt by European leaders struggling to cope with the migrants who make the journey to their shores.
In his invitation letter ahead of the summit, European Council president Donald Tusk proposed the creation of "regional disembarkation platforms" outside the bloc where migrants could be processed, separating refugees from economic migrants.
He warned "there are voices in Europe and around the world claiming that our inefficiency in maintaining the external border is an inherent feature of the European Union, or - more broadly - of liberal democracy".
Talks on the issue are set to begin over Thursday night's working dinner but could stretch into the early hours of Friday if consensus cannot be found.
Mr Tusk also struck a pessimistic tone about transatlantic relations, following the stormy G7 summit in Canada, after which Donald Trump lashed out at host Justin Trudeau.
The European Council president said: "Despite our tireless efforts to keep the unity of the West, transatlantic relations are under immense pressure due to the policies of President Trump."
He added: "It is my belief that, while hoping for the best, we must be ready to prepare our Union for worst-case scenarios."