Boris Johnson on Tuesday faced claims of plagiarism, after copying a spoof version of a scene from the hit Christmas film "Love Actually" as part of his election campaign.

Britain's prime minister is seen in a social media clip taking the part of actor Andrew Lincoln, who turns up to profess his undying love for Keira Knightley by showing her flashcards.

One of the cards Johnson shows a householder in his "Vote Conservative Actually" clip says: "With any luck, by next year we'll have Brexit done (if Parliament doesn't block it again).

"Your vote has never been more important, the other guy could win, so you have a choice to make between a working majority or another gridlocked hung parliament."

The clip has been seen more than one million times since it was posted on Johnson's Twitter account on Monday evening, two days before Britain votes in its third election in four years.

But a main opposition Labour party candidate, Rosena Allin-Khan, had already posted her version of the scene on November 22, as part of her re-election campaign.

"With any luck next year I'll be your MP," the candidate for the Tooting constituency in south London tells a man on the doorstep, as Christmas carols play on portable speakers.

"And our country won't be run by these muppets," she adds, showing another card with photographs of Johnson, members of his government and US President Donald Trump.

Allin-Khan, a doctor in Britain's state-run National Health Service (NHS), on Tuesday called on her followers "not to retweet Boris Johnson's copycat version of #ElectionActually".

A third online parody of the scene has Johnson as the householder answering the door to Trump, who tells him: "With any luck by next year you'll be selling off the NHS."

Actor Hugh Grant, who played a British prime minister in the 2003 romantic comedy and has been campaigning for the pro-EU Liberal Democrats, was asked about the parody on BBC radio.

"I thought it was quite well done, very high production values but clearly the Conservative party have an awful lot of money," he said.

"Maybe that's where the rubles went?" he added, referring to a report into Russian funding of the Tories that Johnson's government refuses to release.

"But I did notice that one of the cards from the original film that he didn't hold up was the one where Andrew Lincoln held up a card saying, 'Because at Christmas you tell the truth'.

"And I just wonder if the spin doctors in the Tory party thought that was a card that wouldn't look too great in Boris Johnson's hands."