“Europe is facing unprecedented threats at our borders,” Mr Sunak said as he prepared to travel to a summit in Moldova, which neighbours Ukraine.
Highlighting Russia’s invasion of Ukraine and the rise in “organised immigration crime”, the prime minister said, “we cannot address these problems without Europe's governments and institutions working closely together”.
“In every meeting, every summit, every international gathering like this, the security of our borders must be top of the agenda. The UK will be at the heart of this international effort to stop the boats and defend our national security.”
Mr Sunak will also announce a new deal on intelligence sharing with Bulgaria to crack down on criminal gangs involved in migration.
He will start negotiations on a new returns agreement with Moldova, which would allow the UK to send back foreign nationals found to be in violation of immigration law. The summit comes as a similar deal with Georgia enters into force.
The meeting is of the European Political Community, which brings together leaders from inside and outside the EU and has been seen as closely linked to French president Emmanuel Macron.
His predecessor Liz Truss attended its first meeting in the Czech Republic last October. A 2024 summit is scheduled to be hosted by the UK.
As nearly 50 European leaders head for the summit, Mr Sunak will meet with Spanish prime minister Pedro Sanchez and Polish prime minister Mateusz Morawiecki.
Support for Ukraine and Moldova in the face of Russian aggression is expected to be on the agenda, with Mr Sunak and Mr Morawiecki set to hold a roundtable discussion on security.
In March, foreign secretary James Cleverly announced £10m for Moldova, earmarked for economic and governance reforms, including the energy sector.
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