Donald Trump has been dealt a major blow after an appeals court unanimously ruled he has no immunity from charges of plotting to overturn his 2020 US presidential election defeat.

The ruling sets the stage for a criminal case leading up to the November 2024 election, for which Trump is the Republican frontrunner candidate.

Trump has vowed to appeal the decision, which was handed down on Wednesday morning (AEDT) by a three-judge panel of the US Court of Appeals District of Columbia Circuit.

The ruling rebuffs his attempt to avoid a trial on charges of undermining US democracy and the transfer of power when he lost the 2020 election to Joe Biden.

“We cannot accept that the office of the presidency places its former occupants above the law for all time thereafter,” the three-judge panel wrote.

The court concluded that any executive immunity that may have shielded Trump from criminal charges while was president “no longer protects him against this prosecution”.

On his Truth Social platform, Trump said it was a “nation-destroying ruling” that “cannot be allowed to stand”.

A Trump campaign spokesman said the ruling “threatens the bedrock of our republic”.

“Without complete immunity, a president of the United States would not be able to properly function,” spokesman Steven Cheung said.

It is one four criminal cases Trump faces, and one of two alleging interference in the 2020 election.

The case will remain paused until at least Monday (local time) to give Trump time to appeal to the US Supreme Court.

His lawyers argued that former presidents were entitled to sweeping legal protections and could not be criminally prosecuted for official actions unless first impeached by the House of Representatives and removed from office by the Senate.

Trump was impeached twice by the House but each time Senate Republicans cast sufficient votes to acquit him of the charges.

Judges homed in on the broad nature of Trump’s claim at a January 9 hearing, questioning a Trump lawyer about whether even a president who ordered military commandos to assassinate a political rival could escape criminal prosecution without initial action by Congress.

The panel wrote in its ruling that giving Trump immunity in this case would give presidents “unbounded authority to commit crimes that would neutralise the most fundamental check on executive power – the recognition and implementation of election results”.

The judges concluded there was no “functional justification” for giving former presidents full protection from federal prosecution even over actions related to their formal responsibilities.

Trump has repeatedly voiced his immunity claim on the campaign trail and social media, warning that his future administration could prosecute President Joe Biden, his likely opponent in the November election, if he returned to the White House.

The indictment brought by independent special counsel Jack Smith accuses Trump of using false claims of voter fraud to pressure state lawmakers, Justice Department officials and then-vice president Mike Pence to thwart the certification of the election results.

Trump has pleaded not guilty to four felony counts and accused prosecutors of a politically motivated effort to damage his campaign.

The immunity argument was previously rejected by US District Judge Tanya Chutkan in December, prompting Trump to appeal.

His appeals have already delayed the start of his trial, which had been scheduled to begin on March 4.

Chutkan has removed that date from the court calendar and not yet set a replacement date.

If Trump wins November’s election, he could seek to pardon himself or direct the Justice Department to shut down the case.

“We cannot accept former President Trump’s claim that a president has unbounded authority to commit crimes that would neutralise the most fundamental check on executive power – the recognition and implementation of election results,” the judges said.

“Nor can we sanction his apparent contention that the executive has carte blanche to violate the rights of individual citizens to vote and to have their votes count.”

In addition, they said that Trump’s stance “would collapse our system of separated powers by placing the president beyond the reach of all three branches.”

“Former president Trump lacked any lawful discretionary authority to defy federal criminal law and he is answerable in court for his conduct,” the panel of judges wrote.

Their scathing opinion describes the former president as using his seat of power to “unlawfully overstay his term as president and to displace his duly elected successor”, all which would violate “generally applicable criminal laws”.

-with AAP