US President Donald Trump has threatened the BBC with a $1 billion lawsuit over the "defamatory, malicious" editing of a speech he gave just before the 2021 US Capitol riots, according to a letter obtained by AFP.

Trump's lawyers gave the British broadcaster a deadline of Friday to fully retract the documentary containing the edit, apologise and "appropriately compensate" the president "for the harm caused."

If the BBC fails to comply "President Trump will be left with no alternative but to enforce his legal and equitable rights... including by filing legal action for no less than $1,000,000,000 (One Billion Dollars) in damages," it said.

"The BBC is on notice. Please govern yourself accordingly."

BBC to 'review' Trump's letter

The director general of the BBC announced his resignation on Sunday over the row, after accusations that a documentary by its flagship Panorama programme last year had edited a speech by Trump in a misleading way.

The BBC said earlier on Monday that it would "review" the letter from Trump's legal team. It also issued a public apology for the editing.

Trump's supporters rioted at the US Capitol on January 6, 2021, protesting his 2020 US presidential election defeat by Democrat Joe Biden.

But the letter from his legal team said that the BBC edit gave a "false, defamatory, malicious, disparaging, and inflammatory" impression of what he said in his speech outside the White House.

'Overwhelming financial and reputational harm'

"Due to their salacious nature, the fabricated statements that were aired by the BBC have been widely disseminated throughout various digital mediums, which have reached tens of millions of people worldwide," the letter said.

"Consequently, the BBC has caused President Trump to suffer overwhelming financial and reputational harm."

A spokesperson for Trump's legal team confirmed that a letter had been sent to the BBC but did not give further details.

"The BBC defamed President Trump by intentionally and deceitfully editing its documentary in order to try and interfere in the presidential election," the spokesperson said in a statement to AFP.

"President Trump will continue to hold accountable those who traffic in lies, deception, and fake news."