Former Business Secretary Peter Mandelson will warn that the U.K. might struggle in trade negotiations if it leaves the European Union because it hasn’t had to conduct any such talks for four decades. Mandelson, who was an EU Trade Commissioner from 2004 to 2008, will use a speech in London on Tuesday to set out the arguments for staying in the 28-nation bloc, warning that a British exit would mean “years of uncertainty.” He’ll also question how long it would take Britain to negotiate new trade treaties with all its partners. “Striking British-only Free Trade Agreement deals would be harder than Brexiters think,” Mandelson will say, according to extracts of his speech released in advance. “After years in the EU, the U.K. has no real trade negotiating capacity, it would first have to be rebuilt.” The vote on whether Britain should leave the bloc it joined in 1973 is scheduled to be held on June 23, with polls suggesting the result could go either way. Prime Minister David Cameron, the Conservative Party leader, is at the head of the campaign to stay inside the EU and is focusing on the dangers of leaving. Mandelson, who served in Tony Blair’s Labour governments, will warn that leaving would see the U.K. forced to accept EU regulation while no longer having a say over it. “In return for market access, we would be required to continue to accept many EU norms and standards,” he’ll say. “As a result, we would have left the EU in order to assert our national sovereignty only to find that, as a condition of access, we did not have independence from EU regulation after all.”