The result of a nationwide referendum has been overturned for the first time in modern Switzerland's history.
The poll, held in February 2016, asked the country's voters whether married couples and co-habiting partners should pay the same tax.
Voters rejected the proposal, with 50.8% against and 49.2% in favour.
But the supreme court has now voided the result on the grounds that voters were not given full information, and the vote must be re-run.
The information provided to the electorate was "incomplete" and therefore "violated the freedom of the vote", the court ruled.
Switzerland's direct democracy system frequently allows citizens to take part in decision-making.
During the referendum campaign, the Swiss government told voters just 80,000 of married couples were paying more tax than couples living together.
The true figure was almost half a million, the government later said.
"Keeping in mind the close result and the severe nature of the irregularities, it is possible that the outcome of the ballot could have been different," the court's statement says.
The Christian Democratic Party lodged an appeal against the result in June 2018, disputing the accuracy of the government's statistics.
Welcoming the court's decision, the party said the ruling was a "boost for the political rights of Swiss voters".
In contrast, those who oppose the ruling argue it could undermine Switzerland's political system, paving the way for frivolous appeals against referendums.
Under the Swiss system of direct democracy, a proposal needs 100,000 signatures before going to a referendum.