Fifty-two percent of voters chose to leave the European Union in a referendum a little more than a year ago, defying the recommendation of both major parties at the time.

But the referendum results specified neither a timetable for the break nor priorities for the future relations with the continent. That has set off complicated debates within each of the major parties. Until now, Labour’s leaders have sought to dodge the question while letting Conservatives feud among themselves.

Both parties are now promising to preserve as much as possible of European free trade rules, which are vital to the British economy. But on immigration — an issue that fired up the referendum vote for an exit — both parties are also pledging to impose new restrictions on the free movement of people, a requirement for membership in the single market.

European negotiators have resisted any suggestion that Britain might get to keep its borders open for trade but closed to migrants. Guy Verhofstadt, the European Parliament’s coordinator on Britain’s exit from the European Union, called that idea “a fantasy” when the government floated a version of it this month.

By staking out a notably softer and more gradual approach, Labour has now set up the clearest contrast yet between the two parties.

“Labour would seek a transitional deal that maintains the same basic terms that we currently enjoy,” Mr. Starmer wrote in the column published Sunday. “That means we would seek to remain in a customs union with the E.U. and within the single market during this period. It means we would abide by the common rules of both.”

Beyond this transition period, he wrote, Labour would seek “a strong and lasting new relationship with the E.U. – not as members, as partners.” Such a relationship “must extend far beyond trade and security to include education, science, technology, medicine and culture” and retain “the benefits of the customs union and the single market,” he wrote.

But, Mr. Starmer added, the proposed “transitional period” should not be a “never-ending purgatory,” because that would not tackle “central issues the referendum exposed – in particular the need for more effective management of migration, which Labour recognize must be addressed in the final deal.”