Leaving the EU would hurt Britain’s jobs and torpedo its global financial clout.
That is the message from pro-Europeans, who have launched their “in” campaign, ahead of a referendum on UK membership.
The campaign is headed up by Lord Stuart Rose, a former boss of retailer Marks and Spencer.
“Do we remain part of the largest free-trade market on the planet, or do we walk away, perhaps risking jobs, creating uncertainty in our economy?” asked Rose.
“Do we lead the world, or continue to lead the world by leading in Europe. Or do we risk diminishing our influence on the world stage by turning our backs on Europe?”
British Prime Minister David Cameron is seeking to renegotiate his country’s relationship with the bloc.
Opinion polls suggest voters are split, with the Greek debt crisis and the migrant influx thought to be encouraging some voters to go for a Brexit.
Rival “out” campaigns have already been launched. One is supported by Nigel Farage, the head of UKIP.
EU critics claim Britain’s membership is holding back its economy, while the bloc lurches from crisis to crisis.
The referendum is due to take place before the end of 2017.