Donald Trump does not have absolute immunity from civil suits seeking damages over his alleged incitement of the January 6 Capitol attack, the US justice department said in a court filing that could have profound implications for complaints against the former president.

In an amicus brief in a case brought by two US Capitol police officers and joined by 11 House Democrats, the justice department said Trump could be held liable for physical and psychological harm suffered during the attack despite his attempts to seek blanket protections.

“Speaking to the public on matters of public concern is a traditional function of the presidency,” read the 32-page brief to the US court of appeals for the DC circuit. “But that traditional function is one of public communication. It does not include incitement of imminent private violence.”

The justice department stressed that it was not weighing in on whether the lawsuit had made a plausible argument that Trump’s speech immediately before the January 6 attack incited thousands of his supporters to storm the Capitol in an effort to stop certification of Joe Biden’s election win.

But the department said that because actual incitement of imminent private violence – the key legal standard – would not be protected by presidential immunity, the appeals court should reject his contention that he had absolute immunity from civil litigation.

“No part of a president’s official responsibilities includes the incitement of imminent private violence,” the brief said. “By definition, such conduct plainly falls outside the president’s constitutional and statutory duties.”

The justice department opinion comes after the appeals court asked the government to offer its position while it considered whether Trump was acting within the confines of the office of the presidency when he urged his supporters to “fight like hell” and march on the Capitol.

The sensitivity of the case – the potential impact on other civil suits against Trump that could have implications for presidential immunity – meant the department took several months and made two requests for a month’s extension before finalising its response.

In siding against Trump’s position that he enjoyed “categorical immunity”, the justice department said it agreed with a lower-court ruling that the first amendment to the constitution did not allow Trump to evade liability in the January 6 suit.

The lawsuit was filed under a statute, enacted after the civil war in response to Ku Klux Klan insurrections across the south to stop Black people voting, which allows for damages when force or intimidation are used to prevent government officials carrying out their duties.

The amicus brief comes as the justice department controversially continues to defend Trump’s claim of absolute immunity in a defamation case brought by the writer E Jean Carroll, who accuses Trump of raping her in New York in the mid-1990s. Trump has said “it never happened” and said Carroll is not his “type”.

Responding to that case, the department argued that while Trump’s comments were not appropriate, they came when he was president. Responding to a reporter’s question about the allegation, the department said part of a president’s responsibility was “to be responsive to the media and public”.