The Trump administration did not give a sufficient explanation for its plan to ask the question, the court ruled.

Writing for the majority, chief justice John Roberts said the commerce department’s explanation was ‘incongruent’ with its priorities and decision-making process, and ordered the case to be reconsidered by a lower court.

However, the mixed ruling does not give a definitive answer on whether the question could be added in future. The commerce department could still be able to proceed with the question if it is able to provide alternative reasoning. The timing may prove difficult as the Census Bureau needs to finalise and begin printing the census questionnaire, although the deadline is unclear.

The decision to reinstate the question of citizenship on the census for the first time since 1950 was announced by commerce secretary Wilbur Ross, who oversees the Census Bureau, in March 2018. At the time, Ross determined the question was necessary to "provide complete and accurate census block level data".

The proposed question was met with controversy from a number of states, civil rights groups, businesses and industry organisations, over fears it would negatively impact response rate, particularly in areas with high immigrant populations, and lead to inaccurate data.

The case was referred to the Supreme Court after federal courts, including New York City district judge Jesse Furman, blocked the inclusion of the question. 

In today’s decision, the Supreme Court partially upheld Furman’s decision, ruling that the district court was warranted in sending the case back to the Census Bureau for further explanation of why the question was needed.