Agencies must offer "genuine justifications for important decisions, reasons that can be scrutinised by courts and the interested public", Roberts 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."

Roberts said a district judge was right to send the issue back to the Commerce Department for a better explanation.

A string of lower-court judges found that Commerce Secretary Wilbur Ross violated federal law and regulations in attempting to include the question on the census. They starkly rebutted his claim that the information was first requested by the Justice Department to enforce the Voting Rights Act, which protects minorities, and they noted his consultations with hard-line immigration advocates in the White House beforehand.

What happens next was not immediately clear, because the department had said it must know by the end of the US summer whether the question can be added.

The case was the most debated Trump administration initiative to reach the high court since last year's 5-4 decision upholding the President's ban on certain travellers from a group of mostly Muslim countries.

Thursday's ruling marked the first major setback for Trump in a ruling at the Supreme Court, although both of Trump's appointees to the court, Neil Gorsuch and Brett Kavanaugh, voted in favor of his administration. Roberts was joined by liberal Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

There was even more at stake in this ruling, and the debate was filled with partisan politics. Opponents of the question called it a Republican ploy to scare immigrants into not taking part in the population count.

Critics argued that adding a citizenship question would discourage millions of people, particularly undocumented immigrants, from filling out the census. This could then be used to justify funding cuts in some areas (because the population would seem smaller than it is) and threaten voter rights.

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An undercount estimated by census officials of more than 8 million people would most affect states and urban areas with large Hispanic and immigrant populations, places that tend to vote for Democrats.

The decennial count of the nation's population determines the size of each state's congressional delegation, the number of votes it receives in the electoral college, and how the federal government allocates hundreds of billions of dollars.

Challengers included Democratic-led states and civil rights and immigrant rights organisations.

The lower court judges have said Ross violated administrative law and the enumeration clause of the Constitution by proposing to ask the citizenship question of each household. Those are the issues that were at stake at the Supreme Court.

But discoveries in the case since the court heard oral arguments in the spring raised new issues.

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Judges in Maryland and New York said they would consider new allegations regarding the claim that Ross's actions violated equal-protection guarantees and was part of a conspiracy to drive down the count of minorities.

The information came from the files of a deceased Republican political operative. Thomas Hofeller, who had been in touch with some census officials at the start of the Trump administration, wrote a memo that said adding the question might help Republicans and white voters in subsequent redistricting decisions based on the census data.

The Washington Post, Reuters