Washington DC - An executive order put out by President Donald Trump requiring proof of citizenship on voter registration forms has been struck down by a US judge.

District Judge Colleen Kollar-Kotelly declared that Trump's executive order was unconstitutional because the president does not have the authority to make such a change to voter registration procedures.

"The first question presented in these consolidated cases is whether the president, acting unilaterally, may direct changes to federal election procedures," Kollar-Kotelly wrote in the ruling.

"Because our Constitution assigns responsibility for election regulation to the States and to Congress, this Court holds that the President lacks the authority to direct such changes."

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Trump's executive order was issued in March on the basis of "preserving and protecting the integrity of American elections," and required the Election Assistance Commission to require documentary proof of US citizenship in its national mail voter registration form.

The Campaign Legal Center, which represents a number of clients involved in the lawsuit, declared victory after the decision.

"This federal court ruling reaffirms that no president has the authority to control our election systems and process," CLC President Trevor Potter said in a statement on social media.

The Trump administration slapped back, however, and confirmed they will challenge the ruling in a higher court, arguing that the president had acted within his executive powers in issuing the order.

"President Trump has exercised his lawful authority to ensure only American citizens are casting ballots in American elections," White House Deputy Press Secretary Abigail Jackson told Fox News.

"This is so commonsense that only the Democratic Party would file a lawsuit against it," she said. "We expect to be vindicated by a higher court."