SEATTLE — Seattle’s wealthiest would become the only Washington state residents to pay an income tax under legislation approved by the City Council, a measure designed as much to raise revenue as to open a broader discussion about whether the wealthy pay their fair share.

The council voted unanimously Monday to impose a 2.25 percent tax on the city’s highest earners. Personal income in excess of $250,000 for individuals and over $500,000 for married couples filing joint returns would be taxed.

The measure is certain to face a court challenge from opponents who call the proposal illegal, unconstitutional and a waste of taxpayer money. If passed, city leaders are likely to keep expanding and increasing the tax over time, they said.

The council supported “an illegal income tax that has no hope of taking effect and will waste taxpayer resources on litigation the city is sure to lose,” said Jason Mercier of the Washington Policy Center,

Supporters of the new tax say the city’s economic growth and prosperity have created significant wealth and opportunity, but it has also exacerbated the affordable housing crisis. Washington is one of seven states without a personal income tax, and a state law passed in 1984 prohibits a county or city from levying a tax on net income.