The Conservative government has promised an in-out referendum by the end of 2017, with recent polls showing the public split neck-and-neck on the issue despite all the main parties campaigning to stay in.
Former interior minister Alan Johnson launched his party's campaign to stay in the 28-member bloc in Birmingham, central England, arguing that Britain would be less safe on its own.
"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 said.
Britons will be asked whether they want to "Remain" in the bloc or "Leave" after the electoral commission ruled that a "Yes" or "No" question could favour those who want Britain to stay in the EU.
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Labour has been a solid supporter of the EU since the premiership of Tony Blair, but that was thrown into doubt with the election of veteran leftist Jeremy Corbyn as party leader in September.
Johnson also said that British workers, businesses, universities and scientists will all be "damaged and diminished" in the event of a "Brexit".
"There is nothing patriotic about condemning this country to isolation," he added.
Prime Minister David Cameron has pledged to campaign to stay in, but only if he secures EU reforms on issues such as welfare payments for migrants and the balance of power between Brussels and national parliaments.
The Labour campaign is separate to the cross-party, pro-EU campaign Britain Stronger in Europe.