Former US President Donald Trump's trial scheduled for March may be postponed because the Supreme Court has rejected a request to expedite a ruling on his immunity claims.

Trump is due to stand trial starting on March 4 over charges he attempted to overturn the result of the 2020 presidential election.

But Trump has been claiming immunity, saying former presidents cannot face criminal charges for conduct related to their official responsibilities.

He first filed the claim with a district court. But as the court rejected it, he appealed to the US Court of Appeals for the District of Columbia Circuit.

The top prosecutor in the case, Special Counsel Jack Smith, asked the Supreme Court to leapfrog the appeals court as a special measure, hoping to avoid any delay in the trial.

But the top court on Friday turned down Smith's request, allowing the appeals court to rule on the issue as normal.

US media say it is likely the first trial will be postponed. March 4 is a significant day because it is the day before Super Tuesday, one of the most important dates in the presidential election process. Observers have said the trial would have an impact on Trump's election campaign.