U.K. Prime Minister Theresa May said Britain would make a definitive break with the European Union while still seeking to negotiate the best possible access to the bloc’s common market.

“Often people talk in terms as if somehow we are leaving the EU, but we still want to kind of keep bits of membership of the EU,” Mrs. May said in an interview with Sky News. “We are leaving. We are coming out,” she said, adding that London still “wants the best possible deal” in terms of trade.

Mrs. May reiterated earlier comments that the U.K. would have control over its borders once outside the EU. She declined to say whether she would prioritize curbing immigration over market access.

EU leaders say the U.K. must allow free movement of people from across the EU if it wants access to the tariff-free single market. Many British lawmakers say membership in the single market is crucial for Britain’s economy.

Mrs. May has faced renewed pressure over the past week from critics who have questioned whether the British government has clear aims in Brexit negotiations, which are due to start by the end of March.

Ivan Rogers, the U.K.’s top diplomat in Brussels, on Tuesday resigned unexpectedly and wrote in a letter to staff that he hoped colleagues would continue to challenge “muddled thinking.”

“Our thinking on this isn’t muddled at all,” Mrs. May said in Sunday’s interview. “We will be outside the European Union but we will get the best possible deal for the U.K. in terms of our trading relationship with the European Union.”

Tim Farron, leader of the pro-EU Liberal Democrats, said Mrs. May was leading the country toward a so-called hard Brexit that he said would leave the U.K. poorer and more divided. “Reckless plans to leave the single market would deal a huge blow to jobs, investment and the public finances, meaning less funding for services like the overstretched NHS,” Mr. Farron said in a statement, referring to the National Health Service.

Leave Means Leave, a pro-Brexit campaign group, said it welcomed Mrs. May’s words. “When we leave single market our ability to trade with EU and access single market will continue,” the group said via its Twitter account.

Mrs. May said she would lay out more details about her Brexit plan in the coming weeks.

(WSJ)