WASHINGTON — US President Donald Trump declared on Monday that he was annulling pardons issued by former president Joe Biden to shield Trump critics from retribution, claiming that they were invalid because his Democratic predecessor had supposedly used an autopen for his signature.

It was unclear what, if any, authority Trump has to void presidential pardons issued by his predecessor. It is extremely rare for pardons to be revoked in US history.

The pardons "are hereby declared void, vacant, and of no further force or effect, because of the fact they were done by autopen," Trump posted on his social media account Truth Social.

However, he provided no evidence either of the use of an autopen or his contention that it would invalidate the signature.

An autopen is a mechanical device that is used to replicate a person's authentic signature. A pen or other writing implement is held by an arm of the machine, which reproduces a signature after a writing sample has been fed to it. Autopens aren't the same as an old-fashioned ink pad and rubber stamp or the electronic signatures used on PDF documents.

US presidents have long used autopens, including to sign bills into law.

Biden issued pardons to former senior Republican lawmaker Liz Cheney and other members of the congressional committee that had investigated the Jan 6, 2021 attack on the Capitol by Trump's supporters and multiple attempts by Trump to overturn the 2020 election in which he lost.

The Biden pardons, issued at the end of his presidency, were effectively a blanket immunity to shield the lawmakers from Trump's repeated promises that he would take revenge against them if he won the 2024 election.

Asked by reporters on Monday whether everything Biden signed with an autopen should be voided, Trump said, "I think so. It's not my decision, that'll be up to a court."

But he said on Truth Social that the committee members "should fully understand that they are subject to investigation at the highest level".

Experts who spoke to Axios media outlet pointed out that even if Trump were to lose a court challenge over the move, the damage would already have been done, with those he targets being dragged into stressful legal battles.

Agencies via Xinhua