Leaving the European Union will leave Britain more exposed to terrorist attack, former home secretary Alan Johnson is warning as he launches Labour's campaign for the UK to remain in the 28-nation bloc.
At a launch event in Birmingham, Mr Johnson will say the attacks in Paris have underlined the need for Britain to stand "shoulder to shoulder" in the fight against terrorism.
"The first duty of any government is to keep our country safe and I firmly believe that leaving the EU would fail that test," he is expected to say.
"From the European arrest warrant to cross-border data sharing on terrorists, the speed of our response is vital.
"The lesson from Paris is clear: to tackle terrorism we must stand shoulder to shoulder with our allies in Europe. The security of Britain is inextricably linked to staying in Europe."
Mr Johnson was chosen by former acting leader Harriet Harman to head up the distinct Labour campaign to keep Britain in the EU in the forthcoming referendum
The ex-postman who rose to one of the highest offices in the land is regarded as having broad popular appeal and will be looking to maximise support for EU membership among Labour voters.
His campaign will operate quite separately from the main cross-party "in" campaign headed by former Marks and Spencer chief executive, Lord Rose.
Mr Johnson will say: "With challenges like the refugee crisis, terrorism and Russian aggression on the EU's borders, Britain is stronger when working with our allies, committed to peace, democracy and international law.
"When this vote comes, it will be a choice between staying in the European Union or leaving and Labour will defend the rights of working people, by campaigning to keep Britain in Europe."