SEATTLE (Reuters) - Washington state Governor Chris Gregoire will announce her support on Wednesday for legislation that would legalize gay marriage in the state, a source who has been informed of her decision told Reuters on Tuesday.

The expected announcement will come just days before the Washington state legislature was scheduled to convene for its next session.

A group of Democratic legislators have said that during the session they will introduce a law to legalize gay marriage. Democrats hold majorities in both chambers of the Washington state legislature.

A spokesman for Gregoire, a Democrat who has long supported gay rights, declined to comment on the governor's plans.

The governor's office issued a media advisory saying only that Gregoire would speak to supporters of gay marriage on Wednesday morning and intended to "address marriage equality" during those remarks.

"The speculation is that she'll support marriage equality and we are looking forward with great anticipation to her speech," said Josh Friedes, director of marriage equality for Equal Rights Washington.

"She has demonstrated great leadership in LGBT civil rights issues and we are confident she'll continue this tradition of leadership," Friedes said.

In May 2009, Gregoire signed a bill putting before the voters a measure granting domestic partners the same rights as married couples as long as they did not conflict with federal law.

That ballot measure, known as Referendum 71, was approved by voters later in 2009.

Six U.S. states currently allow same-sex marriage: New York, Massachusetts, Connecticut, Vermont, New Hampshire and Iowa. Gay marriage is also legal in the District of Columbia.

(Additional reporting by Mary Slosson in Los Angeles; Writing by Dan Whitcomb)

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