Citizens could be less safe if the UK has to leave key policing agreements after Brexit, Prime Minister Theresa May has warned EU leaders.
The UK says EU negotiators are blocking agreement on tracking terrorists and sharing information after March 2019.
The EU says once the UK becomes a "third country" it cannot have the same access to policing initiatives.
Mrs May urged EU leaders to "consider what is in the best interests of the safety of your citizens and mine."
Speaking at a dinner in Brussels the prime minister said obstructing UK participation in law enforcement schemes would mean their ability to fight terrorism would be reduced.
"We would no longer be able to share real-time alerts for wanted persons, including serious criminals," she said.
"We would be able to respond less swiftly to alerts for missing people on either side of the Channel and reunite them with their loved ones.
"Our collective ability to map terrorist networks across Europe and bring those responsible to justice would be reduced.
"That is not what I want and I do not believe it is what you want either," the prime minister said.
She said EU leaders should tell their negotiators to allow the UK to continue to take part in schemes like the Prum mechanism for sharing DNA profiles, the Second Generation Schengen Information System - a database of "real time" alerts about certain individuals - and the European Criminal Records Information System.