Britain's most senior police officer warned on Thursday that the public could be put at risk if the UK leaves the European Union (EU) without reaching an agreement.
Her warning came weeks ahead of the British parliament's vote on the Brexit deal reached between the British government and the EU.
Metropolitan Police Commissioner Cressida Dick told local media that a no-deal Brexit would threaten access to EU-wide criminal databases which will make it harder to extradite people from abroad.
She also said that the Met was talking to other police forces across Europe about contingency arrangements if needed.
Britain is due to leave the EU on March 29 next year.
Dick said Britain's policing cooperation with the EU was based on a framework of "legal instruments" which would have to be replaced after its exit.
But she admitted that a no-deal scenario would make this very difficult to do in the short term.