Washington DC - Donald Trump said Friday that all documents signed by autopen under Joe Biden were "terminated," in a legally uncertain move as the president escalated a favorite attack line against his predecessor.

Trump has often sought to drum up outrage over Biden's alleged use of autopen to sign pardons, executive orders, and other documents – accusing Biden of being too senile to govern while in power.

"Any document signed by Sleepy Joe Biden with the Autopen, which was approximately 92% of them, is hereby terminated, and of no further force or effect," Trump said on social media.

"I am hereby cancelling all Executive Orders, and anything else that was not directly signed by Crooked Joe Biden, because the people who operated the Autopen did so illegally."

Various autopen systems have been used by previous presidents, but Trump has said their use under Biden proves the then-president was mentally incapacitated and not in control of the White House.

Biden was 82 when he left office, while Trump is 79 and due to leave office in January 2029.

Conservative legal commentator Ed Whelan said on social media that Trump was free to revoke executive orders whether or not Biden personally signed them.

"But he doesn't have the same freedom with respect to 'anything else' (e.g., bills enacted by Congress, pardons) that Biden directed be signed by autopen."

The Justice Department in 2005 said the president does not need to sign a bill by hand and can direct an official "to affix the president's signature to such a bill, for example by autopen."

In 2011, The New York Times reported that Barack Obama had become the first president to sign a bill by autopen while in Europe. Paper versions are still sometimes flown to the president for signing.

In his last days in office, Biden issued pardons for people targeted by Trump – including Biden's own son, lawmakers who probed Trump, a military general who had criticized Trump, and the country's top Covid expert.