Brexit on Theresa May's terms could see the UK lose access to an important database of information on terrorists and criminals which would poses a "direct threat" to "national security", the Liberal Democrats have said.
Under the Prime Minister's plans to end the jurisdiction of the European Court of Justice within the UK, police may lose access to the second generation Schengen Information System (SIS II) database, Nick Clegg, the party's Brexit spokesperson said.
SIS II is a database of ''real time'' alerts about individuals of interest, such as wanted or missing people, to EU law enforcement agencies.
It contains information on thousands of people wanted under the European Arrest Warrant, as well as alerts on suspected foreign fighters.
The SIS II is used by countries across the borderless Schengen area, while it includes special arrangements for countries like Britain that are not part of the passport-free zone.
Countries that are part of the Schengen Area but not part of the European Union - including Norway, Switzerland, Liechtenstein and Iceland - also have access to the SIS II.
UK police and security services queried the database over half a billion times in 2016 - equivalent to 16 checks a second.
The Lib Dems said access to the sensitive information held on SIS II is limited to countries which abide by the rulings of the European Court of Justice (ECJ), meaning that under Mrs May's plans the UK could lose its ability to query the database after Brexit happens in 2019.
Mr Clegg said: "Theresa May's extreme approach to Brexit will have the direct consequence of severing our ties to a fantastically useful weapon in our armoury against terrorism.
"By refusing to accept a role for the European Court of Justice in policing this European-wide database, she has ruled out our future participation it.
"It is hard to overstate the importance of this database. We check it 16 times a second, looking for security threats that have been flagged to us by other European countries. And we use it to tell other countries to stop and question people who we think are potential terrorists.
"This is Euroscepticism gone mad. If she fails to back down, Theresa May's approach to Brexit poses a direct threat to our national security."
However, the Conservatives hit back at the former deputy prime minister's comments: "The Lib Dems are peddling nonsense - we've made clear that under Theresa May security cooperation will be an important priority in the negotiations, and it's not in the EU's interest to lessen that relationship."
The Government's own Brexit white paper highlighted the value of the SIS II information, noting that from April 2015 to April 2016 "over 6,400 foreign alerts received hits in the UK, allowing UK enforcement agencies to take appropriate action, whilst over 6,600 UK-issued alerts received hits across Europe".
The document set out ministers' hopes to "negotiate the best deal we can with the EU to co-operate in the fight against crime and terrorism", adding that "public safety in the UK and the rest of Europe will be at the heart of this aspect of our negotiation".