Brexit on Theresa May's terms could see the UK lose access to an important database of information on criminals and terrorists, former deputy prime minister Nick Clegg has 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.
Mr Clegg, the Liberal Democrat Brexit spokesman, warned that Mrs May's plans pose "a direct threat to our national security".
SIS II is a database of ''real time'' alerts about individuals of interest 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.
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."
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".
Mr Clegg told BBC Radio 4's Today programme the Tories needed to offer concrete answers on how they would keep the "devastatingly effective" crime fighting tool.
He said: "There's a huge difference between saying we want a deal and making a deal feasible and possible.
"This is where I think the Conservatives need to stop, dare I say it, sort of patronising people by constantly saying, 'Yes, we will sort it out, don't worry your pretty little heads. We want to do it, they want to do it as well'.
"Unless they have concrete answers about how you square the circle about wanting to have access to data which can only be legally both uploaded and accessed unless all the participating countries abide by rulings of the European Court of Justice, however much people keep saying they want this to happen, it won't happen in reality."
Home Secretary Amber Rudd said she did not "underestimate" the importance of intelligence-sharing with the EU and insisted that the UK would be able to negotiate a deal to retain key crime-fighting measures such as the European Arrest Warrant and SIS II.
She also moved to reassure European allies that Britain would remain a "strong partner in defence and security" once negotiations begin, amid concern from German Chancellor Angela Merkel that Europe cannot completely depend on the UK.
Ms Rudd told BBC Radio 4's Today programme: "We can reassure Mrs Merkel that we want to have a deep and special partnership so that we can continue to maintain European-wide security to keep us all safe from the terrorists abroad and are trying to be nurtured in our country.
"I don't underestimate the fact that access to a number of EU instruments like the one he (Nick Clegg) was referring to, SIS II, are important to keep us safe.
"But I believe that we can negotiate a position with the European Union whereby we continue to have the membership of those - SIS II, European Arrest Warrant, Europol - so that we continue to share information and keep both sides safe."