(Adds start of U.S.-UK meeting, comments)
WASHINGTON, July 24 (Reuters) - Britain wants to ensure business relations with the United States are not disrupted and will seek negotiations on an "ambitious" trade agreement with the United States, UK trade secretary, Liam Fox, told Trump administration officials at the launch of discussions on Monday on post-Brexit planning.
Fox and his U.S. counterpart, Robert Lighthizer, jointly chaired the first meeting of a U.S.-UK trade and investment working group, which will start work on a bilateral trade agreement between the countries after Brexit.
Britain is not free to enter into new trade deals until it has left the EU in 2019. It has indicated, however, that it wants to get legal documents in place to ensure that such things as flights and data flows between the countries are not interrupted.
"The immediate priority is to give businesses on both sides of the Atlantic certainty and confidence," Fox told the working group in opening remarks made available to the press. "Early discussions will focus on providing commercial continuity for US and UK businesses as the UK leaves the EU," he added.
On Tuesday, as part of Britain's Brexit transition campaign, Fox is due to address U.S. lawmakers on Capitol Hill and launch a report detailing British trade and investment with 435 congressional districts in the United States.
Trade between the United States and Britain is about $200 billion annually.
"We have begun to look at our continuity agreements to maintain as open and flexible relationship as we can," Fox told the American Enterprise Institute think tank earlier on Monday.
"We're looking at bilateral ways, while we're still members of the European Union, to achieve trade liberalization on a number of fronts on science and technology, and we are looking to scope out the future free trade agreement with no preconceptions attached to that," he added.
Fox defended Britain's decision to leave the bloc and said those still hoping to change the 2015 British referendum result, which supported Brexit, "are dreaming."
U.S. President Donald Trump has said his administration would work hard to get a quick bilateral trade deal done.
Trade was a major issue during the Brexit campaign when former U.S. President Barack Obama said Britain would have to go "to the back of the queue" for a deal if it voted to leave the EU. (Reporting by Lesley Wroughton; Editing by Tom Brown, Bernard Orr)