British Prime Minister Theresa May said on Tuesday she would now lead negotiations with the European Union, shifting the focus of the Brexit department towards preparing for Britain's departure from the bloc.

A special unit within May's office has played an increasing role in Brexit talks during recent months, and Tuesday's announcement formalises that shift in responsibility.

"I will lead the negotiations with the European Union, with the Secretary of State for Exiting the European Union (Dominic Raab) deputising on my behalf," May said in a written statement to parliament.

"DExEU (Department for Exiting the EU) will continue to lead on all of the government's preparations for Brexit: domestic preparations in both a deal and a no-deal scenario, all of the necessary legislation, and preparations for the negotiations to implement the detail of the Future Framework."

Questioned about the change, May's Europe adviser Oliver Robbins said he doubted the change would see her meeting EU chief negotiator Michel Barnier to negotiate directly.

"The key interlocutor for Mr Barnier is the secretary of state," Robbins told a parliamentary committee.

"What the prime minister means is that she meant all along that the overall strategy and conduct of these negotiations she regards very much as her personal responsibility now with the secretary of state very close at hand."

Former Brexit minister David Davis, who resigned in protest earlier this month after May announced her Brexit negotiating strategy, was reported by newspapers to be frustrated at the increasing influence of May's office, and Robbins in particular, on the negotiations.