The US Supreme Court has dealt a blow to the Trump administration's efforts to place a question on citizenship in the 2020 census.

Donald Trump had wanted to include a question asking those answering the census, which takes place every 10 years, on whether or not they were legally documented as US citizens. Some had feared that adding the citizenship question would deter undocumented citizens from participating in the census, leading to a predicted undercount of about 6.5m people. Such a reduced number could drastically undercut representation when congressional districts are allocated in 2021, and affect how hundreds of billions of dollars in federal spending are distributed.

The administration claimed their reason for the question was to help enforce the Voting Rights Act of 1965 in order protect minority voters. That logic has been questioned by three federal judges already, who determined that Wilbur Ross, the US commerce secretary who is in charge of the revised 2020 census form, was lying about the question's intentions, as well as who had requested it. The judges found that Ross had requested the Justice Department to add the question, and not, as he had claimed, the other way around.

In May, evidence discovered from a now-deceased Republican strategist's computer suggested that the real reason for the question was that it would “be advantageous to Republicans and non-Hispanic whites."

In Thursday's decision, the court refused to rule on the case at this point, instead sending it back to a lower court. The ruling means the question might not be decided on before 2020 census ballots are printed for distribution.

Writing for the majority, Chief Justice John Roberts Jr. said the explanation for asking whether or not a person is a US citizen offered by the administration was inadequate. It's a surprising turn for the conservative judge, who voted with the progressive side on the issue.

“The reasoned explanation requirement of administrative law, after all, is meant to ensure that agencies offer genuine justifications for important decisions, reasons that can be scrutinised by courts and the interested public,” the chief justice wrote. “Accepting contrived reasons would defeat the purpose of the enterprise. If judicial review is to be more than an empty ritual, it must demand something better than the explanation offered for the action taken in this case.”

When the decision was alerted on smart phones outside the Homestead Detention Centre in Florida, so-called volunteer witnesses cheered.

“It’s wonderful! That’s just one way the Trump administration has been trying to chip away at the rights of those who are the neediest among us,” Nanette Bartels, a 68-year-old education consultant from Cutler’s Bay, said.

“Oh this is wonderful, this is so good! I think the census is really important, it’s a national issue not a political issue,” Michelle Cuartas, a 36-year-old non profit worker from Hollywood said. “A question of citizenship immediately invoked fear and the idea you’re being categorised for your legal status in this country. I can understand that there’s fear, because of the way the Trump administration is handling immigration, and the anti-immigrant sentiment.”