Theresa May will insist Britain is unconditionally committed to the defence and security of Europe during informal talks with leaders from across the EU.
On a visit to Estonia, the Prime Minister will say it is in the interests of every country in the bloc to confront the growing terror threat, illegal migration and increased Russian aggression together.
She will tell European counterparts the UK's role in Europe's defence has "never been more vital" and point out Britain has the largest defence budget, top-class security and intelligence services, and is a key player in Nato.
Mrs May, along with French president Emmanuel Macron and Estonian prime minister Juri Ratas, will pay a visit to troops based in the country as part of Nato's effort to reassure eastern European nations fearful of Russia's increasing assertiveness.
About 800 UK soldiers have been stationed in Tapa since April leading a Nato battle group alongside Estonian and French armed forces.
Mrs May will offer UK expertise on combating cyber threats from the National Cyber Security Centre (NCSC) when she meets EU leaders at the Tallinn digital summit.
The recent spate of major cyber attacks across Europe, including an assault on the NHS, shows the need for closer co-operation on tackling the danger to financial systems and the public sector, she will say.
Speaking ahead of the summit, 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.
"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."
Britain is seeking a bespoke deal on security links with the EU under proposals to maintain co-operation on efforts to fight terrorism and serious crime after Brexit.
The Government wants a "comprehensive" new framework that would be underpinned by a new treaty.
It has also made clear the UK is ready to continue contributing troops, equipment, expertise and money to EU operations, and to align foreign policy with Brussels where appropriate.