LONDON—Prime Minister Theresa May said the U.K. intends to leave the European Union’s single market, outlining a plan for a definitive break from the bloc and answering the biggest open question about her vision for Britain’s future.

In her most detailed speech yet, Mrs. May on Tuesday said the U.K. wouldn’t seek a “half-in, half-out” relationship with the EU. Once it is out, she said, Britain would no longer abide by the bloc’s immigration rules or be subject to jurisdiction of the European Court of Justice, which the EU insists are requirements for unfettered access to its market of 440 million consumers.

At times conciliatory and other times tough, the British leader described an independent U.K. with full control over its borders that would nonetheless maintain a close and friendly trading relationship with the EU. “We will continue to be reliable partners, willing allies and close friends,” she said.

In comments that appeared to soothe investors, she said she would give lawmakers, a majority of whom backed staying in the EU, a vote on the final deal, adding she wanted a transition period to smooth the shift to new relations.

Sterling rose by as much as 2.8% against the U.S. dollar to $1.228 during the speech, having plummeted to a 31-year low on Monday after newspapers suggested she would take an uncompromising approach on leaving the single market.