British Prime Minister Theresa May's bid to reassert her dwindling authority has been marred by a calamitous keynote speech interrupted by repeated coughing fits, a prankster and even letters of her slogan falling off the stage.

May had wanted to use the Conservative Party conference to bring her divided party together and pitch herself as the only person able to deliver Brexit and keep Labour leader Jeremy Corbyn out of power.

She started by apologising for her botched bet on a snap June election which stripped her party of its majority in Parliament, then pitched a revitalised British Dream for which she proposed fixing broken markets and uniting the country.

But her flow was interrupted by British comedian Simon Brodkin, who handed her a P45 letter, a document given to employees when they leave their job. The document had been signed by her ambitious Foreign Secretary Boris Johnson.

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Then May began a coughing fit and was repeatedly forced to take drinks of water, even coughing into her glass, and was proffered a lozenge from Chancellor of the Exchequer, Philip Hammond. While she was speaking, several letters fell off the slogans behind her on the stage.

May, 61, won standing ovations for pressing on with the hour-long address, in which she took a more personal tone - saying she did not mind being called the Ice Maiden and describing her great sadness at not having children. May sought to make the case for free markets and fiscal prudence.

She tried to compete with Labour on its pledges to voters, offering £2 billion to build cheaper houses, proposing a cap on what she called rip-off energy prices and to ease the burden of student debt.

Many in the audience said she won them over. But Nigel Farage, the former leader of Ukip, said May was so useless that if she remained as leader then Corbyn would soon be in power.

- Reuters, AAP