Actor Hugh Grant has been revealed as the driving force behind some epic trolling of outgoing British PM Boris Johnson, as he prepared to finally fall on his sword.

The scandal-ridden Mr Johnson had spent days defying pleas to quit from a parade of Conservative MPs – and an precedented wave of resignations from his cabinet and other senior roles.

But late on Thursday (Australian time) he eventually conceded the inevitable, announcing he would resign.

As he did, it was to the wild sounds of Yakety Sax, the famous Benny Hill Show theme song, echoing loudly across the crowds of protesters, supporters and assembled journalists at the Houses of Parliament.

As it turns out, the Love Actually star was at least partly responsible for the surprise tune. Earlier on Thursday, Grant asked activist and anti-Brexit campaigner Steve Bray if he “had the music to hand”.

Tweet from @HackedOffHugh

“Morning [Mr Bray],” Grant tweeted.

“Glad you have your speakers back. Do you by any chance have the Benny Hill music to hand?”

Mr Bray did – and made sure to play it at full volume. It can even be heard in the background of plenty of British news broadcasts, as they interview MPs (including Mr Johnson loyalists) caught up in the fray.

Mr Johnson wants to stay until a new prime minister is chosen – which could take months.

“Today I have appointed a cabinet to serve, as I will, until a new leader is in place,” he said in his resignation speech outside his Downing Street office, watched by allies and his wife Carrie.

“I know that there will be many people who are relieved and perhaps quite a few who will also be disappointed. And I want you to know how sad I am to be giving up the best job in the world. But them’s the breaks,” he added, making no apology for the events that forced his announcement.

Tweet from @MissRiley88

There were cheers and applause as Mr Johnson began his speech, while boos rang out from some outside the gates of Downing Street.

After days of battling for his job, Mr Johnson had been deserted by all but a handful of his closest allies after the latest in a series of scandals.

“It was a short and bizarre resignation speech which didn’t mention the word resign or resignation once. There was no apology, no contrition,” Conservative MP Andrew Bridgen said.

“There was no apology for the crisis his actions have put our government, our democracy, through.”

Details of the process to elect a new Conservative leader will be announced next week.

A snap YouGov poll on Friday (Australian time) indicated that defence minister Ben Wallace was the favourite among Tory members to replace Mr Johnson, followed by junior trade minister Penny Mordaunt and former finance minister Rishi Sunak.

While Mr Johnson is determined to remain caretaker PM throughout the process, opponents and many Conservatives said he should leave immediately and hand over to his deputy, Dominic Raab.

Former Tory PM John Major said it was “unwise and maybe unsustainable” for him to remain in office when he could still exert its powers.

“For the overall wellbeing of the country, Mr Johnson should not remain in Downing Street – when he is unable to command the confidence of the House of Commons – for any longer than necessary to effect the smooth transition of government,” Mr Major said in a letter released to media.

Keir Starmer, leader of the main opposition Labour Party, said he would call a parliamentary confidence vote if the Conservatives did not remove Mr Johnson immediately.

Support for Mr Johnson had evaporated during one of the most turbulent 24 hours in recent British political history, epitomised by finance minister Nadhim Zahawi, who was appointed to his post only on Tuesday, calling on his boss to resign.

Mr Zahawi and other cabinet ministers went to Downing Street on Thursday, along with a senior representative of those MPs not in government, to tell Mr Johnson the game was up.

Initially, Mr Johnson refused to go and seemed set to dig in. He sacked Michael Gove –- a member of his top ministerial team who was one of the first to tell him he needed to resign – in a bid to reassert his authority.

But, as a slew of resignations poured in – including that of Michelle Donelan who he had appointed education secretary late on Tuesday – it became clear his position was untenable.

“You must do the right thing and go now,” Mr Zahawi tweeted.

-with AAP