A federal judge on Thursday ordered US President Donald Trump to sit for a deposition in his US$10 billion defamation lawsuit against the BBC by December 11.

The order from US Magistrate Judge Enjolique Lett came as lawyers for the British broadcaster complained that Trump had delayed his appearance. Charles Tobin, the lawyer for the BBC, said that with two months left in the discovery period, “we still haven’t been able to depose the plaintiff”.

He noted there was an agreed-upon date of October 16, but that Trump had cancelled it for a “matter of national importance”. Tobin said the Republican president was in fact campaigning in Georgia that day. “He was participating in party politics and prioritised that over sitting for the deposition,” Tobin said.

Lett said Trump must sit for a deposition by December 11, or face sanctions up to dismissal of his lawsuit – unless the discovery period was extended past December 18.

Discovery, where lawyers for both sides exchange documents and other evidence, is scheduled to end on December 18, and the BBC’s lawyers want to make sure they have Trump’s deposition before that deadline.

Lett said only the presiding judge assigned to the case can alter the trial schedule, and the defence said it would be open to scheduling the deposition at a later time if the trial schedule was also moved back.

Trump, who has often avoided depositions, filed his lawsuit against the BBC last December. He accused the publicly funded broadcaster of defamation as well as deceptive and unfair trade practices related to a documentary, .

The BBC acknowledged that filmmakers spliced together quotes from different sections of the speech Trump delivered to his supporters on January 6, 2021, before the storming of the US Capitol, to make it seem he was directly urging violence. The broadcaster apologised over the editing, and the head of the BBC and its top news executive both resigned. But the BBC has denied Trump’s claims that it defamed him.

To bolster its defence, the broadcaster subpoenaed former special counsel Jack Smith earlier this month. Trump’s lawyers argued, in a joint filing asking for a hearing on the matter, that there was no good reason for Smith, who is not a party to the case, to appear. The parties were expected to address the issue later in Thursday’s hearing. “Smith lacks any personal knowledge regarding the issues in this matter,” they argued in the filing.

“He was not present when President Trump’s January 6, 2021, speech … was made, nor did he participate in its preparation. Smith does not have any personal knowledge regarding President Trump’s state of mind at the time the Speech was given.”Trump’s side also argued that “Smith’s testimony would be duplicative”, because he published a lengthy report on the events of January 6 and also testified for many hours. “The entirety of his testimony is publicly available,” it argued.

The BBC countered, in the same filing, that there were few more knowledgeable sources about the events of January 6 than Smith, who studied and investigated them for years.

“It is difficult to imagine a more authoritative source on these issues than Mr Smith, who spent years gathering evidence on those topics with the force of the Justice Department and the power of the grand jury behind him,” the BBC’s lawyers argued.

“No one forced plaintiff to file this lawsuit and to dispute that his speech on January 6 incited the violence that followed at the Capitol.

“But as long as he chooses to maintain the case, he cannot commit the fundamental injustice of preventing the BBC from obtaining the evidence it needs to robustly defend itself on every element of his claim.”