Leaving the EU will lead to big tariffs on UK exports like cars, whisky and textiles - and higher prices in the shops because of import tariffs, Peter Mandelson has warned.
In his first intervention in the referendum campaign, the former Labour trade minister and EU trade commissioner accused Leave campaigners of "trying to sell people a fantasy".
He also said leaving the EU would be damaging for British business, and the alternative trade arrangements proposed by Brexit campaigners would be dangerous for Britain.
"Losing the EU’s preferential trading benefits in foreign markets could mean new tariffs of 10, 20%, or sometimes even more, on key UK exports such as cars, machine goods, whisky and textiles," Lord Mandelson said.
"The UK would also potentially have to raise its own tariffs on imports from these markets as they would no longer be covered by WTO-compliant agreements."
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Lord Mandelson argued trade is central to the referendum choice because Britain trades nearly half its exports into the EU and through the EU’s global trade deals we gain access to export markets round the world.
He said the EU conducts all trade policy as a single powerful bloc on behalf of its members, including Britain, and leaving the EU would mean losing domestic and international trading opportunities unless or until they were replaced with new ones - which would take time.
"Brexiters cannot argue that we are weakened in the EU as it is, but would suddenly be strong enough to dictate terms if we left," he said.
"For every politician who saw the pragmatic case for dealing with the UK, there would be another who had little doubt that the UK must not be given a quick or easy ride.”
And endorsing the warning made in a Government dossier this week on what withdrawal from the EU would mean, Lord Mandelson said: "The negotiation would mean years of uncertainty and in the worst case scenario a return to paying EU tariffs while a final deal on an FTA was struck."
While Lord Mandelson becomes the first senior Labour figure to take the fight to the Leave campaign, Tory eurosceptics will continue their battle for the lifting of a ban on Cabinet ministers campaigning to leave the EU being given Government documents by civil servants.
The Cabinet Secretary, Sir Jeremy Heywood, will appear before the Public Administration Committee of MPs, chaired by leading eurosceptic Tory MP Bernard Jenkin, and be forced to explain his reasons for the ban.
And at the weekly Cabinet meeting in 10 Downing Street, Cabinet "outers" such as hardliner Iain Duncan Smith - who has pledged to defy the Prime Minister and order his civil servants to hand over key papers - may challenge David Cameron to lift the ban.