A federal judge in Seattle blocked Donald Trump’s administration on Thursday from implementing an executive order curtailing the right to automatic birthright citizenship in the US, calling it “blatantly unconstitutional”.

US district judge John Coughenour at the urging of four Democratic-led states issued a temporary restraining order preventing the administration from enforcing the order, which the Republican president signed on Monday during his first day on office.

The order has already become the subject of five lawsuits by civil rights groups and Democratic attorneys general from 22 states, who call it a flagrant violation of the US constitution.

“Under this order, babies being born today don’t count as US citizens,” the Washington assistant attorney general Lane Polozola told Judge John Coughenour at the start of a hearing in Seattle.

In a press conference outside the court after the restraining order was issued, Polozola said: “This is step one but to hear the judge from the bench say that in his 40 years as a judge, he has never seen something so blatantly unconstitutional, sets the tone for the seriousness of this effort.”

Polozola and the other challengers argue that Trump’s action violates the right enshrined in the citizenship clause of the constitution’s 14th amendment that provides that anyone born in the United States is a citizen.

Trump in his executive order directed US agencies to refuse to recognize the citizenship of children born in the US if neither their mother nor father is a US citizen or legal permanent resident.

In a brief filed late on Wednesday, the US justice department called the order an “integral part” of the president’s efforts “to address this nation’s broken immigration system and the ongoing crisis at the southern border”.

The lawsuit filed in Seattle has been progressing more quickly than the four other cases brought over the executive order. It has been assigned to Coughenour, an appointee of the Republican former president Ronald Reagan.

The judge potentially could rule from the bench after hearing arguments, or he could wait to write a decision ahead of Trump’s order taking effect.

Under the order, any children born after 19 February whose mothers or fathers are not citizens or lawful permanent residents would be subject to deportation and would be prevented from obtaining social security numbers, various government benefits and the ability, as they get older, to work lawfully.

More than 150,000 newborn children would be denied citizenship annually if Trump’s order is allowed to stand, according to the Democratic-led states.