A prankster, who has previously targeted US President Donald Trump and former FIFA president Sepp Blatter, interrupted British Prime Minister Theresa May's keynote speech at her annual conference today.

British comedian Simon Brodkin, who uses the stage name Lee Nelson, handed her a P45 letter, a document given to employees when they leave their job, to May as she gave her speech to Conservative supporters.

As he was bundled out of the hall, May briefly stopped speaking before the party faithful stood to applaud and cheer her.

As security agents escorted him out of the conference hall, Brodkin quipped that Foreign Secretary Boris Johnson - who is widely believed to have ambitions to be prime minister himself - had told him to hand the P45 to May.

"Boris told me to hand her a P45," he told Reuters. "He didn't tell me why. He wanted me to do it."

Writing on his Twitter feed shortly afterwards, Brodkin said: "Hi @BorisJohnson, I gave Theresa her P45 just like you asked."

The comedian interrupted a media conference by President Trump at his Scottish golf course last year by rolling golf balls towards him emblazoned with swastikas and in 2015 showered Blatter with fake paper money at a briefing with reporters.