PM Theresa May is to warn EU leaders to be wary of "hostile states like Russia" and pledge the UK will stay committed to European security after Brexit.
In Brussels for a summit, she will say it is crucial that European countries work together to "protect our shared values and ideals".
She will also discuss Brexit with European Council President Donald Tusk.
Last week Mr Tusk said the UK must show more progress on the "divorce bill" if trade talks were to begin this year.
On Friday, Mrs May is expected to stress the need for a unified approach to security as the UK leaves the EU.
She will say: "From agriculture in Ukraine to the tech sector in Belarus – there is a huge amount of potential in the Eastern neighbourhood that we should nurture and develop.
"But we must also be open-eyed to the actions of hostile states like Russia which threaten this potential and attempt to tear our collective strength apart.
"This summit highlights the crucial importance of the European countries working together to protect our shared values and ideals.
"The UK may be leaving the EU but we are not leaving Europe, and we are unconditionally committed to maintaining Europe's security."
Last week Mrs May accused Russian President Vladimir Putin's government of trying to "undermine free societies" and "sow discord in the West".
BBC Brussels reporter Adam Fleming said Friday's summit was about highlighting the EU's commitment to its partners in the East – countries like Ukraine and Azerbaijan.
He said the prime minister would use it to demonstrate that the UK could still contribute to European security after Brexit, for example by spending £100m over five years to fight Russian disinformation campaigns.
Brexit is not on the official agenda but Mrs May will meet Mr Tusk for talks, weeks before the next EU summit in December.
At that summit, EU leaders will decide whether enough progress has been made on issues like the financial settlement, the Irish border and citizens' rights to move on to the next phase of Brexit talks – on trade and a potential transition deal.
Earlier this week, the cabinet agreed that the UK should offer to pay more money to the EU – thought to be up to £40bn – but not before the EU agrees to begin talking about a new trade deal.
Last week Mr Tusk said the EU was "ready" to move onto the next phase of Brexit talks at the December 14/15 summit but the UK must first show more progress on the outstanding issues.
The UK voted to leave the EU in June 2016, and served the EU with formal notice of Brexit in March 2017. This began a two-year countdown to the UK's departure day which will be in March 2019.
