The UK will be less safe if it fails to strike a post-Brexit security deal with the EU, Britain's top counter-terrorism officer has said.
Neil Basu told the BBC's Newscast podcast a deal was "incredibly important for the safety and security of our country".
He said he was hopeful of a better security and law enforcement agreement than the UK currently has.
The government said the safety and security of citizens was a priority.
Talks between the UK and EU are ongoing ahead of the 31 December deadline for a deal.
The UK left the EU on 31 January, but continues to follow current EU rules until the end of the year while negotiations take place.
Any deal would need to be ratified by parliaments on both sides.
On security, the UK had wanted to maintain the same access to shared databases that it has now, but the EU says that is not on offer to non-members.
Metropolitan Police Assistant Commissioner Mr Basu told BBC Newscast: "We need to negotiate a security treaty that either retains or improves on the current position that we've got today.
"It's incredibly important for the safety and security of our country. And I know the government gets that. And I know that that is what it's aiming for."
When asked what a no-deal Brexit would mean for UK security, he said: "The country would be less safe in a non-negotiated outcome where a security treaty wasn't forthcoming. That's the bottom line."
Mr Basu said he was keen to know the outcome of negotiations as soon as possible.
"We've made it very clear that we need as much time to negotiate those agreements with our European counterparts as possible, and we can't do that until the negotiations are finished," he said.
"We've been very clear to government right from the start of this process, and I'm confident they have listened to our concerns."
A spokesman for the government said: "The safety and security of our citizens is our top priority, and the UK will continue to be a global leader on security and one of the safest countries in the world.
"We are focused on reaching an agreement with the EU and there is a good degree of convergence in what the UK and EU are seeking to negotiate in terms of operational capabilities."
If it was impossible to reach an agreement, they said, "we have well-developed and well-rehearsed plans in place".
On Monday, Prime Minister Boris Johnson said he was "confident [the UK] will prosper" outside the EU if a post-Brexit trade deal is not agreed with the bloc".
Following the most recent talks in Brussels, the UK's chief negotiator Lord David Frost said there had been "progress", while his EU counterpart Michel Barnier said he wanted "future cooperation to be open but fair" with the UK.