By Jenny Gross

LONDON -- Prime Minister Theresa May on Wednesday defended herself against criticism that she doesn't have a plan for Britain's exit from the European Union, saying during a combative session with lawmakers that her cabinet was united in its approach to leaving.

The U.K. leader, who took over in July after Britain's historic vote to leave the bloc, is facing increasingly loud demands from the opposition and others for details. With four months to go until a self-imposed deadline to formally begin the process of extricating the U.K. from the EU, Mrs. May must also grapple with a coming court ruling that could disrupt her timeline.

"Yes, we do have a plan," Mrs. May told Parliament, standing by her position thatBritain wants access to Europe's tariff-free single market while controlling the number of EU migrants entering the U.K. Beyond that, she declined to spell out more, saying giving a running commentary would ensure the U.K. got the worst possible result from negotiations.

"We are preparing carefully for the formal negotiations. What we want to ensure is that we have the best possible trading deal with the European Union once we have left," she said.

On Tuesday, Mrs. May's office responded angrily to a memo written by Deloitte LLP and published in the Times of London and the British Broadcasting Corp. that said the government didn't have a coherent plan and Britain would need six more months to decide what it wants to achieve from Brexit.

Mrs. May's government called the memo an attempt to drum up business for Deloitte, an accounting and consulting firm. Deloitte later issued an apology and said the memo was written for internalaudiences and wasn't commissioned by the government.

Jeremy Corbyn, leader of the opposition Labour Party, seized on the memo. "Isn't the truth that the government is making a total shambles of Brexit and nobody understands what her strategy actually is?" he asked.

Mrs. May said the "government is getting on with the job. He's not up to the job."

A poll published Wednesday by Ipsos Mori found that 48% of the public think the government is doing a bad job at handling Britain's exit from the EU, and 37% said it is doing a good job. The rest said they didn't know. In the June referendum on whether to leave the bloc, 52% voted to leave.

Foreign Secretary Boris Johnson, in an interview with a Czech newspaper on Tuesday, said the U.K. wanted to maintain free trade with the EU while leaving its customs union, which applies customs duties, import quotas and other tariff barriers on goods entering the bloc

Jeroen Dijsselbloem, the Dutch finance minister, said that vision of Britain's future relationship with the EU was "intellectually impossible."

"Britain and the continent now look at each other with suspicion," Mr. Dijsselbloem said at an event hosted by UBS Group AG in London on Wednesday.

Also this week, Italy's economic development minister told Bloomberg News that the U.K.'s Brexit strategy is chaotic. "Somebody needs to tell us something, and it needs to be something that makes sense," Carlo Calenda said in the interview. "You can't say that it's sensible to say we want access to the single market but no free circulation of people."

Mrs. May faces legal hurdles too. The government is appealing a ruling this month that Mrs. May can't start the process of Brexit without approval from Parliament. If upheld by the Supreme Court, the High Court decision offers lawmakers an opening to disrupt her plans and steer the country toward a relationshipwith the EU that has stronger ties and a more-open immigration policy than Mrs. May has suggested she would pursue.

A fresh twist came after a Supreme Court judge suggested that the parliamentary process involved in triggering Article 50, the start button on the U.K.'s departure from the EU, would be more complicated than initially thought.

Brenda Hale, one of 11 judges due to hear the appeal next month, said it might be necessary for Ms. May's government to first repeal existing legislation before passing a new law authorizing her to set the process of departure in motion.

The case raised "difficult and delicate issues" about Britain's constitutional arrangements, said Ms. Hale, who was speaking in an address to Malaysian law students in Kuala Lumpur earlier this month. The Supreme Court published the speech online on Tuesday.

"Another question is whether it would be enough for a simple Act of Parliament to authorize thegovernment to give notice, or whether it would have to be a comprehensive replacement for the 1972 Act," said Ms. Hale, referring to the European Communities Act, which enshrines European law in the U.K. and which formed a central plank in the plaintiffs' case.

--Georgi Kantchev contributed to this article.

Write to Jenny Gross at jenny.gross@wsj.com

(END) Dow Jones Newswires

November 16, 2016 11:49 ET (16:49 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.