WASHINGTON (Reuters) - The District of Columbia's mayor has signed a $15-an-hour minimum wage into law, a rate adopted by a growing number of U.S. cities and states seeking to battle income inequality.

Mayor Muriel Bowser signed the measure late on Monday. The bill boosts minimum hourly wage to $15 by 2020, with subsequent hikes tied to inflation.

"Three months ago, I said we would take up the #FightFor15 inDC and I am so excited to sign it into law today!" Bowser, a Democrat, said on her Twitter feed.

The bill had been unanimously approved by the city council. It was opposed by business owners who argued that it would raise costs and lead to automation, and would make the city less competitive with neighboring suburbs.

Washington joins California and New York in making $15 the hourly minimum. At least eight cities, including Seattle, have also approved the $15 base.

Supporters had said that Washington's robust economy and growing population meant it could support a higher minimum wage.

The District of Columbia's base wage is $10.50, and will go

up by $1 on July 1 under existing law. The federal minimum is

$7.25 an hour.

President Barack Obama cheered Washington's move and called on Congress to hike the federal base wage. "America deserves a raise," he said in a statement.

The $15 minimum is estimated to raise wages for 114,000

workers, or about 14 percent of the District of Columbia's

workforce, according to an analysis by the non-profit Economic Policy Institute.

(Reporting by Ian Simpson; Editing by Alistair Bell)