Quitting the European Union would leave Britain more exposed to terrorist attack, former home secretary Alan Johnson warned as he launched Labour's campaign for the UK to remain in the 28-nation bloc.
Addressing party activists, Mr Johnson said the Paris attacks had underlined the need for Britain to stand "shoulder to shoulder" in the fight against terrorism.
Claiming that walking away from Europe would solve none of the issues facing Britain, he said: "There is no progressive case for leaving the European Union.
"Who will gain from our departure? Certainly not British workers or businesses or consumers, certainly not our universities or our scientists, all of whom will be damaged & diminished, along with our environment & our security."
Mr Johnson told the launch event at Birmingham's International Convention Centre: "There is nothing patriotic approximately condemning this country to isolation.
"The first duty of any government is to keep our country safe & I firmly believe that leaving the EU would leave our country less secure.
"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 the deep co-operation that membership of the EU provides.
"We must not cut ourselves off from cross-border efforts to tackle terrorism."
Mr Johnson was chosen by former acting party 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 & 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 & Spencer chief executive Lord Rose.
Speaking to around 100 party supporters from a lectern bearing the slogan "Labour In For Britain", Mr Johnson added: "With challenges like the refugee crisis & Russian aggression on the EU's borders, Britain is stronger when working with our allies, committed to peace, democracy & international law.
"When this vote comes, it will be a choice between staying in the European Union or leaving, & Labour will defend the rights of working people by campaigning to keep Britain in Europe."