There would be a risk to the public from a hard Brexit, police chiefs have warned.
Police and Crime Commissioners have written to the Home Secretary to express concern over what withdrawing from the EU will mean for security measures.
They have asked Sajid Javid to confirm the Home Office has developed a contingency plan for a no deal Brexit.
The letter from the Association of Police and Crime Commissioners (APCC) said: "Through discussions with the NCA [National Crime Agency] and NPCC [National Police Chiefs' Council], we understand that considerable additional resource would be required for policing to operate using non-EU tools and that such tools would be sub-optimal - potentially putting operational efficiency and public safety at risk."
It continues: "There are 32 Law Enforcement and National Security Measures [Lens] that are used on a daily basis in an operational policing context.
"Unless the Government is able to negotiate the retention of these measures following the UK's withdrawal from the EU, police and law enforcement agencies face a significant loss of operational capacity.
"As Police and Crime Commissioners, we are increasingly concerned that such a loss of capacity could pose significant risks to our local communities."
The police chiefs also state the Brexit negotiations come at a time when the in-country threat from foreign national offenders targeting the UK from abroad is increasing, and when international co-operation is a key element of the fight against crime.
They further said that with the implementation period unlikely to be known until October this year, the five-month window until the end of March 2019 is "likely to be very challenging".
"We are therefore concerned that a 'no deal' scenario could cause delays and challenges for UK policing and justice agencies," the letter states.
The Commissioners have asked Mr Javid to ensure the need to retain Lens tools is prioritised, that he works closely with the APCC, NPCC and NCA to get a detailed understanding of the potential risks, and consider the financial provisions and extra resources needed to support contingency planning in the event of no security treaty being achieved.
The cross-party Brexit working group has requested a meeting with the Home Secretary to discuss "preparations, contingencies and the financial implications of post-Brexit policing".