If we quit the European Court of Justice, we won't be able to access information which enable officials to stop and question over 50 terrorism suspects a day at our borders.
The Schengen Information System (SIS II) is an EU-wide database on organised criminal and terrorist suspects across 28 countries, including 35,000 people wanted under a European Arrest Warrant. It includes alerts on suspected 'foreign fighters' - people who have travelled to Syria and elsewhere to fight for ISIS.
Nick Clegg, the Liberal Democrats former Deuty prime Minister, has challenged Theresa May to answer three vital questions:
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.
Read for yourself more about the dangers highlighted by Nick Clegg