The billionaire founders of e-commerce company Amazon Jeff and MacKenzie Bezos have pledged $2.5 million to the fight for equal marriage rights for gay couples in the US state of Washington.

Washington’s state legislature passed equal marriage laws earlier this year.

The legislation was due to come into effect in June, but was delayed as opponents submitted more than 200,000 signatures to force the measure through a referendum this November.

The anti-equality group Preserve Marriage Washington submitted almost 232,000 signatures for Referendum 74, which is nearly twice the number of required signatures to put the measure to a public vote.

While every state ballot on equal marriage so far undertaken in the US has seen voters block marriage rights for gay couples, the left-leaning think-tank Public Policy Polling said last month that voters in Washington may be the first to approve equal marriage. Their polling indicated 51 percent supported equal marriage, with 42 opposed.

Equal marriage proponents raised nearly $1m in June alone with major donations from Microsoft’s co-founder Bill Gates and CEO Steve Ballmer.

Now, Amazon’s founders have pledged to effectively double the money raised so far with a $2.5m gift, the New York Times reports.

Zach Silk, campaign manager for Washington United for Marriage, said on Thursday: “To get this from a straight, married couple sends a powerful message that marriage is seen as a fundamental question of fairness.”

The Bezoses have declined to speak publicly about the gift.

The NY Times said the Bezoses responded to an email from a gay former employee in Amazon.com’s first days asking whether they would donate $100,000 or $200,000 to the cause, similar to amounts donated by Microsoft figures. The response said: “This is right for so many reasons. We’re in for $2.5 million. Jeff & MacKenzie.”