A US federal judge has struck down Donald Trump's executive order targeting law firm Perkins Coie as a violation of the country's constitution's protections of free speech and due process and castigated him for "settling personal vendettas."

District judge Beryl Howell's ruling represented the broadest rebuke yet for the Republican president's pressure campaign against law firms that he has accused of "weaponizing" the justice system against him and his political allies.

It was also the first ruling by a judge deciding the legal merits of any of the several directives Trump has aimed at law firms that have handled legal challenges to his actions, represented political adversaries or employed lawyers who have taken part in investigations of him.

Howell, in a sharply worded, 102-page opinion, said Trump's executive order was an attack on foundational principles of American jurisprudence and the role lawyers play in ensuring the fair and impartial administration of justice.

"Settling personal vendettas by targeting a disliked business or individual for punitive government action is not a legitimate use of the powers of the US government or an American President," Howell wrote.

Perkins Coie said it welcomed the judge's ruling and was grateful to "those who spoke up" in support of the firm's lawsuit.

"As we move forward, we remain guided by the same commitments that first compelled us to bring this challenge: to protect our firm, safeguard the interests of our clients, and uphold the rule of law,” a spokesperson for the law firm said.

Howell, based in Washington, barred federal agencies from enforcing Trump's March 6 order against Perkins Coie. The judge had previously issued a temporary restraining order blocking enforcement of key provisions of Trump's directive.

The Justice Department can appeal Howell's order.

Perkins Coie, a 1,200-lawyer firm founded in Seattle, represented the campaign of 2016 Democratic presidential nominee Hillary Clinton, who Trump defeated in his first presidential run.

Trump's executive order sought to restrict Perkins Coie's lawyers from accessing government buildings and officials, and it threatened to cancel federal contracts held by the firm's clients.

"In a cringe-worthy twist on the theatrical phrase 'Let’s kill all the lawyers,'" Trump's executive order "takes the approach of 'Let’s kill the lawyers I don’t like,' sending the clear message: lawyers must stick to the party line, or else," Howell wrote.

Three other major law firms – WilmerHale, Jenner & Block and Susman Godfrey – also sued the administration to block the executive orders Trump issued against them. Other judges have temporarily blocked those orders while the cases proceed.

Nine rival firms – including Paul Weiss, Latham & Watkins; Skadden Arps; and Willkie Farr – have reached deals with Trump that averted punitive actions, pledging a combined total of nearly $1 billion in free legal services to advance causes he supports.

Trump's order accused Perkins Coie of "dishonest and dangerous activity."

It also said Perkins Coie "racially discriminates" in its hiring — referring to the firm's diversity policies. Trump and his allies have portrayed such policies as discriminatory against white people. Trump's order also criticized the firm's work representing Clinton's campaign. (Reuters)