LONDON — The leader of Britain’s opposition party, Jeremy Corbyn, tried on Monday to position Labour as the standard-bearer of a “soft” exit from the European Union, narrowing the maneuvering room for Theresa May, the country’s already weakened prime minister.

In a much-anticipated speech, Mr. Corbyn proposed that Britain remain in a customs union with European Union member states when it leaves the bloc, to soften the economic shock of an abrupt exit.

He made that position conditional on Britain being able to take part with the European Union in negotiating trade deals with other countries, a notion Brussels has so far steadfastly resisted.

Mr. Corbyn’s shift on the customs union, which eliminates tariffs and reduces other barriers to trade between members, was immediately welcomed by two major British business organizations — a significant step, since British industry has traditionally seen the Conservatives as the more business friendly party.