BRUSSELS (Reuters) - Michel Barnier, the French politician who will lead EU negotiations with London on Britain's exit from the European Union, plans to tour member states once he takes office on Oct. 1, EU sources said on Tuesday.

"He'll do a round of capitals and gauge the mood," one source said, noting that until Prime Minister Theresa May formally notifies Brussels of the British departure under Article 50 of the EU's Lisbon treaty, negotiations will not start.

Barnier, a conservative former minister and European Commissioner, was appointed by Commission President Jean-Claude Juncker to serve as his chief negotiator with Britain following the Brexit referendum in June.

Barnier, 65, who oversaw financial services regulation for the EU executive until 2014, has already been joined by several staff including a German deputy, trade negotiator Sabine Weyand, and expects to have a "critical mass" of about 10 people on his team within a couple of weeks, sources said.

His staff is likely to reach around 20 in time, though the negotiating process, once Britain has triggered it, will draw in much larger numbers of Commission officials.

One source said Barnier was planning to concentrate his team on negotiating three main areas: disentangling Britain from the EU budget; trade and foreign policy; and issues around the single market, including migration of workers.

It is unclear when formal negotiations will start. May's government is still assessing its objectives and she has said she will not trigger Article 50's two-year countdown to exit before next year. Juncker and other EU leaders say they will not discuss terms until she does, although some governments are keen to give London some idea of where talks might lead.

Barnier wants to hear from the other 27 EU governments what their priorities are. He will coordinate the Commission's work on the detail of any divorce settlement, which will also need the agreement of the Council of the other member states, as well as of the European Parliament.

Some British politicians have complained the EU negotiating line-up, including Barnier, represents interests hostile to Britain. European leaders say they want a deal that keeps Britain close and limits economic damage, but they will put the interests of the EU first.

(Reporting by Brussels bureau; Editing by Mark Trevelyan)