A prankster and comedian has humiliated British Prime Minister Theresa May, offering her a resignation form as she took the podium to deliver a speech at an annual party conference.

Lee Nelson, in an apparent security breach, rose up to the Conservative Party leader to hand her a P45, the tax form issued by an employer when an employee leaves.

Nelson joked that the form was from Boris Johnson, the British foreign secretary who has previously made a bid to lead the UK.

May was further embarrassed as an "f" dropped off the word "for" from a sign behind her which had read: "Building a country that works for everyone".

The four-day Conservative Party conference in the northern city of Manchester is expected to end today.

Britain's exit from the EU, or Brexit, and the economy were among the subjects discussed.

The event comes amid uncertainty over the Brexit deal, with Britons' fearing an agreement to quit the bloc could leave the country worse off financially.

Nelson is known for showering the former FIFA president, Sepp Blatter, in fake money. Blatter was at the time accused of corruption.

Video of Wendesday's prank went viral on social media.

"Theresa May" soon trended on Twitter.

"The next 24 hours have suddenly become the most crucial in Mrs May's political life," said journalist and media personality Andrew Neil.

"On a serious point, MASSIVE security lapse to let prankster with P45 into hall front row by @theresa_may. Very scary," said Robert Peston, an editor with ITV.

Some praised the prime minister's response, however.

"Not a fan of Theresa May at all but her resolve is quite impressive, both today and in general. She's been through quite a lot. #CPC17," said blogger Oliver Norgrove.

"Great that @theresa_may has managed to turn a protester and nearly losing her voice to her advantage #CPC17" said Conservative activist Robert Winfield‏.

Here are some more tweets:

Hi @BorisJohnson, I gave Theresa her P45 just like you asked. pic.twitter.com/gzW0UluDMv — Simon Brodkin (@simonbrodkin) October 4, 2017

Theresa May is living the first verse of Lose Yourself by Eminem in front of our eyes. — Jamie Ross (@JamieRoss7) October 4, 2017

Source tells me ISIS have claimed responsibility for Theresa May's speech — Hussein Kesvani (@HKesvani) October 4, 2017

Credit to Theresa May for somehow battling on.

But P45, lost voice & set collapsing makes this biggest speech fiasco in political history. pic.twitter.com/uCu9drXJKR — Piers Morgan (@piersmorgan) October 4, 2017

So far she's been given a P45 and a cough sweet. She'll get a pizza delivered soon. What a mad conference speech this is. #CPC17 — Pete Wishart (@PeteWishart) October 4, 2017