Washington (AFP) - US trade with Britain will increase "very substantially" post-Brexit, President Donald Trump predicted on Friday as a deadline looms for that country to leave the European Union.

"You know all of the situation with respect to Brexit and the complexity and the problems but we have a very good trading relationship with UK," Trump told reporters at the White House.

"We're continuing our trade and we are going to actually be increasing it very substantially as time goes by."

With Britain due to exit the EU next month, London faces the prospect of losing the benefit of trade deals the EU struck with other economies, which could raise tariffs and other impediments to the country's exports.

But the United States and Britain on Thursday announced two "mutual recognition agreements" to ensure the country will continue to be subject to the same criteria applied to the EU on rules for telecommunications and computing equipment, pharmaceuticals inspections and marine equipment.

In case Britain crashes out of the EU without an agreement on trade relations, London has won agreements with Australia, New Zealand, Chile and Switzerland to ensure mutual recognition of standards in goods trade, providing a bridge to new trading regimes for Britain outside the EU.

Official figures this week showed the British economy posted its slowest growth in six years, amid uncertainty over Brexit.