In a statement to the BBC, a spokesman for Trump’s legal team said the BBC had “intentionally defamed” the president, and with its subpoena request was now “seeking to harass him, his family, and supporters by abusing the deposition process”.

“The BBC is simply trying to distract away from their own obvious liability,” the statement said.

The BBC’s lawyers argue in the latest filing that Trump’s family members had “personal knowledge” of his intentions in delivering the speech, making their testimony and documents relevant to the president’s defamation claim.

Drawing on evidence previously gathered by a US congressional committee, the filing says Donald Trump Jr and Ivanka Trump were present when their father revised and delivered his speech, adding that Jared Kushner drafted a proposed statement for him that would have condemned the use of violence.

The filing asks the court to allow the subpoenas to be served by email and certified mail, saying attempts to serve them by hand were unsuccessful.

A subpoena is an order for someone to answer questions under oath or provide evidence. Failure to comply results in a penalty.

According to court filings, in one instance, a representative for the BBC’s legal team tried to deliver a subpoena for Ivanka Trump and Jared Kushner at their home in Florida, but was refused entry at a police checkpoint and was later told by the Secret Service that the agency could not accept the subpoenas.

Similarly, according to the filing, a representative tried to serve a subpoena for Donald Trump Jr at Trump Tower in New York but was told by the concierge that the person who normally took such documents may have been at lunch.

During another attempt two days later, the representative was told the “legal department” was not in that day, according to the filing.

Trump’s legal team has been approached for comment. The BBC press office declined to comment.