Britain's access to information in Europe-wide security databases is "mission critical" in efforts to protect the public, a senior counter-terrorism officer has said.
Helen Ball, Deputy Assistant Commissioner at the Metropolitan Police, highlighted the issue of returning Islamic State fighters as she discussed law enforcement arrangements with the EU following the Brexit vote.
The future of a number of tools and arrangements has come under scrutiny following the outcome of the referendum in June.
They include: the European Arrest Warrant for speeding up extradition of individuals between member states; the Second Generation Schengen Information System (SIS II) - a database of real time alerts; the European Criminal Records Information System (ECRIS) established for the exchange of information on criminal convictions between member states; and Europol, the EU's law enforcement agency.
Ms Ball told the Lords EU Home Affairs Sub-Committee that usage is "much less in terms of volume" in counter-terrorism policing but it is "very useful to have those structures and mechanisms operating".
She added: "If we were no longer to have access to them, we would be seeking to have nevertheless the ability to share information and intelligence across Europe."
Counter-terrorism units also have a particular interest in data contained in passenger name records - information relating to airline passengers - financial information, the movement of firearms and biometric information such as fingerprints and DNA.
Ms Ball said: "It is mission critical in protecting both the citizens of the UK and citizens of Europe that the UK policing effort is able to access that information.
"I'm not going to say it has to be through a particular formal mechanism, that's for the negotiations to decide."
She said European countries face the "common problem" of the possibility that people who have travelled to Syria to join Daesh - also known as Islamic State -will return.
"We all have faced the problem of our citizens being manipulated, persuaded to travel or persuaded to carry out attacks in their country by Daesh. We are much stronger if we can share the information about that," Ms Ball said.
"If we know that people are travelling across Europe to reach Syria to join Daesh, being able to both track them and to prevent that travel is extremely important."
Around 850 people linked to the UK and regarded as a security threat are believed to have taken part in the Syrian conflict, with just under half thought to have returned to the country.
The threat from international terrorism in Britain currently stands at severe, indicating that an attack is "highly likely".
Ms Ball stressed MI5 and police have "very strong" relationships with their counterparts in Europe.
"We have more reliance on those relationships than we do on access to the databases, nevertheless we wouldn't want to lose access to that information," the officer said.
Ms Ball said use of the European Arrest Warrant in counter-terrorism policing is currently low. However, she added: "As I look into the future I suspect we will have greater reason to use it.
"We mustn't be in a position where a terrorist can think 'OK there's a safe haven where it's going to take a very long time for me to be extradited and come to meet justice'.
"So if it were not to be the European Arrest Warrant, going forward we will want something that means we can bring people to justice very swiftly."