LONDON (REUTERS) - It is more important than ever for the European Union to strike a trade deal with the United States, Secretary of State John Kerry said on Tuesday (July 19), adding there was no way for Britain to strike a trade pact with Washington before leaving the bloc.

Britain voted in a referendum last month to leave the EU but Prime Minister Theresa May's new government has yet to decide on when to trigger formal exit talks.

Speaking in London, Kerry said it would be "physically impossible" to cut a trade deal with the United Kingdom before it leaves the EU, although he said after talks with new Foreign Secretary Boris Johnson that informal talks could begin earlier.

He also stressed that the United States needs a united and engaged Britain as an ally.

"The United States of America depends on a strong United Kingdom. We mean united and it depends also on an engaged United Kingdom," Kerry told reporters at a joint news conference with his newly appointed British counterpart Boris Johnson.