By David Lawder

WASHINGTON (Reuters) - U.S. Trade Representative Michael Froman told his new British counterpart on Monday that the two countries cannot launch negotiations on bilateral trade and investment deals until more is known about Britain's future relationship with the European Union.

The U.S. Trade Representative's office said in a statement that Froman told UK Trade Secretary Liam Fox in a meeting in Washington "that the United States will be prepared to engage in conversations with the United Kingdom about how to develop our trade and investment relationship in the best way at the appropriate time."

Britain has yet to launch separation negotiations with the EU since the June 23 Brexit vote, and little is known about the level of future economic integration between an independent Britain and the trading bloc that will be reduced to 27 countries. At stake are whether UK-EU tariffs will be reimposed, the ability of London-based financial firms to operate on the continent and immigration rules affecting employees.

"As a practical matter, it is not possible to meaningfully advance separate trade and investment negotiations with the United Kingdom until some of the basic issues around the future EU-UK relationship have been worked out," the USTR office said in the statement.

The Obama administration also is evaluating how Britain's departure from the EU will affect negotiations for the U.S-EU Trans-Atlantic Trade and Investment Partnership free trade deal.

Talks have progressed slowly, and Froman has recently expressed concerns that offers must be readjusted to account for the departure of Britain, which accounts for 25 percent of U.S. exports to the bloc and buys 40 percent of U.S. wine exports to the EU.

(Reporting by David Lawder; Editing by Dan Grebler)