The UK is unconditionally committed to the defence and security of Europe, Prime Minister Theresa May will say.
Mrs May is in the Estonian capital Tallinn for an informal meeting of European Union leaders.
She will visit 800 British soldiers who have been based in Tapa since April as part of a Nato battlegroup with French and Estonian troops while in Estonia.
Mrs May will also attend the Tallinn digital summit, where she will call for greater co-operation on cyber threats.
She will be joined French President Emmanuel Macron and Estonian Prime Minister Juri Ratas to visit troops who will be joined by forces from Denmark in the new year.
At the summit, the prime minister will offer to share UK expertise from the National Cyber Security Centre to combat cyber threats.
Speaking ahead of her visit, Mrs May said: “From terrorism to cyber-crime, illegal migration to Russian aggression, the threats we face as Europeans are increasing in their scale and complexity.
“Now more than ever, it is in all our interests to confront them together.
“With the largest defence budget in Europe, a far-reaching diplomatic network, world-class security, intelligence and law enforcement services, and our position at the heart of Nato, the UK’s role in Europe’s defence has never been more vital.
“As we prepare for Brexit, I want to build a bold, new security partnership with the EU.
“A partnership that reflects our shared history, promotes our common values and maintains a secure and prosperous Europe.
She added: “Nato remains the bedrock of our collective security and there is no clearer demonstration of the UK’s unconditional commitment to Europe’s defence than the 800 British troops now in Tapa, leading a Nato battlegroup and standing shoulder to shoulder with their Estonian, French and, soon, their Danish counterparts too.
“We will continue to work with our Nato allies, our European neighbours and the EU to support a future partnership of unprecedented breadth and depth, that will guarantee the security and stability of the continent for generations to come.”