Labour has launched its campaign for the UK to stay in the EU, claiming the country's security depends on its continued membership.
The campaign, led by ex-Home Secretary Alan Johnson, will be run separately to the cross-party, pro-EU campaign.
He said the Paris attacks underlined the need for Britain to "stand shoulder to shoulder with our allies in Europe".
Campaigners for an exit said the EU had not responded quickly enough to terror threats, making the UK less safe.
David Cameron is to hold an in/out vote on the EU by the end of 2017, but an exact date has not yet been set. The prime minister has said he will not set the date until he has secured an agreement from the other EU leaders on his renegotiations.
Mr Cameron has said he wants the UK to stay in a reformed EU, but has not ruled out recommending leaving if he cannot secure the change he wants.
Speaking at Labour's campaign launch in Birmingham, Mr Johnson said there was "no progressive case" for Britain to leave the 28-member bloc and suggested that doing so would expose the country to greater risks.
"There is nothing patriotic about condemning this country to isolation," he said. "The first duty of any government is to keep our country safe and I firmly believe that leaving the EU would fail that test.
"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 added: "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."
Mr Johnson was asked to head Labour's 'In' campaign by acting leader Harriet Harman earlier this year. The former postman and trade union official held a series of Cabinet roles in the last Labour government before leaving frontline politics four years ago.
While the majority of Labour MPs support continued membership of the EU, Jeremy Corbyn is regarded as the most Eurosceptic leader of the party for decades and several trade unions have said they could contemplate campaigning to leave if Mr Cameron's changes erode workers' rights.
The prime minister has outlined four goals for reforming the UK's membership of the EU, including restrictions on benefits for people coming to the UK.
Mr Johnson told activists that Labour's campaign would be focused on "defending the rights of British workers", suggesting that employees, businesses, consumers, students and scientists would all be "damaged and diminished" by exiting the EU.
Vote Leave, one of two umbrella groups campaigning for the UK to leave the EU, said the UK's existing relationship with the EU had made the country "less safe", with control over borders, human rights law and crime-fighting powers ceded to Brussels and EU courts.
It also questioned the EU's ability to tackle the threat from terrorism, saying its 28 members had failed to reach an agreement on sharing watch lists of airplane passengers across Europe - something the UK has demanded immediate action on.
"Pro-EU campaigners want to scare you into voting to 'remain' - but the EU is incapable of responding quickly enough to evolving terror threats," it said. "We will be safer if we take back control rather than carrying on giving even more power to Brussels."
Referendum on the UK's future in the European Union
The UK is to have a referendum by the end of 2017 on whether to remain a member of the European Union or to leave. The vote is being proceeded by a process of negotiations in which the Conservative government is seeking to secure a new deal for the UK.
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