The justices ruled 6-3 to throw out a lower court's decision that had allowed a charge of corruptly obstructing an official proceeding - the congressional certification of President Joe Biden's victory over Trump that the rioters sought to prevent - against defendant Joseph Fischer, a former police officer. 

The justice directed the lower court to reconsider the matter.

Fischer had challenged the obstruction charge, which federal prosecutors brought against him and hundreds of others - including Trump - in January 6-related cases. 

The ruling was a setback for the US Justice Department and Biden's administration and a potential boost for former US president Donald Trump.

Trump, the Republican candidate challenging Democrat Biden in the November 5 US election, was hit with the obstruction charge as part of a four-count criminal indictment in a case brought last year by Special Counsel Jack Smith. 

The charge falls under the 2002 Sarbanes-Oxley Act, a federal law passed after the accounting fraud scandal at now-defunct energy company Enron.

Fischer was accused by prosecutors of charging at police guarding a Capitol entrance during the attack. 

Fischer, at the time a member of the North Cornwall Township police in Pennsylvania, got inside the building and pressed up against an officer's riot shield as police officers attempted to clear rioters, according to prosecutors.

He remained in the Capitol for four minutes before police pushed him out, they said.

Fischer has been awaiting trial on six other criminal counts, including assaulting or impeding officers and civil disorder, while his challenge to his obstruction charge has proceeded.

US District Judge Carl Nichols, a Trump appointee, granted Fischer's request to dismiss the obstruction charge, ruling it applies only to defendants who tampered with evidence. 

The US Court of Appeals for the District of Columbia Circuit reversed that decision, prompting Fischer's appeal to the Supreme Court.

Federal prosecutors estimate that about 250 of the roughly 1400 people charged in the Capitol attack by Trump supporters could be affected by the ruling. 

According to Justice Department data, about 50 January 6 defendants were convicted and sentenced on the obstruction charge with no other felony. 

Of those, about half are currently serving a sentence of incarceration - representing less than 2 per cent of all charged cases.

The charge carries a sentence of up to 20 years in prison if convicted, although January 6 defendants convicted of obstruction have received far lesser sentences.

The legal issue in the case involved how two parts of the obstruction law fit together. 

The first provision prohibits obstructing an official proceeding by destroying "a record, document or other object". 

The second part makes it a crime to "otherwise obstruct" an official proceeding.

The Justice Department argued that Congress included the second provision to give the obstruction law a broad sweep.

About two thirds of the January 6 defendants charged with obstruction also were charged with other felonies.

The Supreme Court heard arguments in the case in April.

After the 2020 election, Trump and his allies alleged it had been stolen from him through widespread voting fraud.

Federal and state election officials and Trump's attorney general said there is no credible evidence the election was tainted.

On the day when Congress met to certify Biden's victory, Trump supporters stormed the Capitol, broke through barricades, attacked police officers, vandalised the building and forced lawmakers and others to flee for safety.

In August 2023, Smith brought four federal criminal counts against Trump in the election subversion case: conspiring to defraud the United States, corruptly obstructing an official proceeding and conspiring to do so and conspiring against the right of US citizens to vote.

with AP