Washington DC - President Donald Trump's attempt to attack law firm WilmerHale with another executive order has been deemed unconstitutional and struck down by a federal judge.

US District Judge Richard Leon's ruling marks the third time in a month that Trump's attempts to undermine law firms that have worked on cases he disapproves have been quashed in the courts.

"The cornerstone of the American system of justice is an independent judiciary and an independent bar willing to tackle unpopular cases, however daunting," Leon wrote in his Tuesday ruling.

"The Founding Fathers knew this!" Leon added, before stating that if he were to permit the executive order, it "would be unfaithful to the judgment and vision of the Founding Fathers."

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WilmerHale employed former Department of Justice (DOJ) special counsel Robert Mueller, who led the investigation into alleged Russian influence on the 2016 election.

Trump's executive order accuses the firm of being "bent on employing lawyers who weaponize the prosecutorial power to upend the democratic process and distort justice," before specifically making reference to Mueller.

"This weaponization of the justice system must not be rewarded, let alone condoned," the executive order alleged, before introducing sanctions against WilmerHale, including barring its employees from federal buildings.

"The court’s decision to permanently block the unlawful executive order in its entirety strongly affirms our foundational constitutional rights and those of our clients," a spokesperson for WilmerHale was cited as saying by the Guardian.

"We remain proud to defend our firm, our people, and our clients," the spokesperson added.