Johnson jokes about UK PM's 'lederhosen'

UK's Prime Minister Theresa May has been ridiculed by politician Boris Johnson after she was photographed wearing a pair of brown leather trousers.

Boris Johnson has ridiculed UK Prime Minister Theresa May for her brown leather trousers. (AAP)

Boris Johnson has poked fun at Theresa May over so-called Trousergate - joking that the British prime minister wears "lederhosen".

The prime minister has come under fire after being photographed for a newspaper magazine feature in a pair of brown leather trousers costing almost STG1000 ($A1687).

Johnson, who has been on the receiving end of barbs from the prime minister and Chancellor Philip Hammond in recent weeks, took the opportunity to have fun at May's expense at a diplomatic reception in London's Lancaster House.

The Brexit-backing foreign secretary highlighted the close trade links between the EU and UK before ridiculing the prime minister.

"Our wonderful PM actually wears lederhosen," he said.

Johnson also highlighted Nigel Farage's links to Donald Trump as he told the assembled diplomats about the UK's export achievements - sending sand to Saudi Arabia, boomerangs to Australia and "Nigel Farage to America".