The European Union is an essential element in countering the threat of terrorism, former cabinet minister David Blunkett is warning.
Lord Blunkett - who was home secretary at the time of the 9/11 attacks - will use a speech on Thursday to say that if the EU had not existed "we would have had to invent it".
He will also say it would be a "calamity" if Britain was forced to abandon its border controls in Calais as a result of a vote for Brexit on June 23.
The Labour grandee - who voted against Britain's membership of the Common Market in the 1975 referendum - becomes the latest political heavyweight to enter the increasingly fraught referendum campaign.
He will draw on his experience of 9/11 to underline the importance of the EU relationships to Britain's security.
"The meetings we held at the justice and home affairs council, at a Europe-wide level after September 11, were illustrative of the importance of the ability to work quickly and effectively with all those who were then part of the union," he will say.
"The European Arrest Warrant, the data sharing improvements, the use of biometrics and yes, improved collaboration on the wider European border all demonstrate the importance of what we call the European Union.
"In simple terms, if we had not had the EU, we would, on these issues, have had to invent it."
Lord Blunkett will also warn that the importance of the agreement which allowed Britain to operate border controls at Calais and at the Eurostar terminals in Paris and Brussels had been "grossly underestimated".
"I am absolutely clear that this agreement could not have been reached had we not built an understanding, worked together as part of and understood that our future was in, the European Union," he will say.
"Were the French to decide to revoke the agreement, which leading French spokespeople have indicated, it would be a calamity for robust and rational border controls.
"This would lead to an increase in asylum claims from people who came to our shores and the disappearance of tens of thousands of people into the illegal economy."
Addressing his own past-Euroscepticism, Lord Blunkett will say: "Forty years ago I voted 'no' to staying in the European Union. So what has changed? The simple answer is 'The world.'"