More than 640,000 people have signed a petition to thank coffee giant Starbucks over its support for equal marriage after opponents tried to launch an international boycott.

The number dwarfs the 25,800 who have signed the National Organization for Marriage’s Dump Starbucks pledge to avoid the coffee chain in future over its support for equal access to marriage in the US state of Washington.

SumofUs.org originally said it would deliver a giant ‘thank you’ card to the company’s Seattle headquarters and give executives “moral support to keep standing up for equality” if the petition reached 40,000 signatures, a mark it quickly surpassed.

The card is due to be delivered to executives today.

Taren Stinebrickner-Kauffman, executive director for SumofUs.org said: “We’ve been stunned by the passionate response to “Thank Starbucks” – our most viral campaign ever.

“We’ve seen in our campaigns that consumers expect corporations to do right by their workers and by their community. We hope that this overwhelming show of support for Starbucks for supporting gay rights will inspire other corporations to keep making similar public statements.”

Zach Silk, Campaign Manager for Washington United for Marriage said: “We knew that people would stand with Starbucks for standing up for marriage equality, but this response is amazing. This kind of consumer support shows why so many Washington companies recognize that all loving, committed couples should have the freedom to marry.

“Washington United for Marriage is a coalition of organizations, congregations, unions, and businesses working together to secure civil marriage for same-sex couples in Washington State.

In January, the Seattle-based hot drink giant said it was “proud” to join other Washington-based employers like Microsoft and Nike in supporting the equal marriage in the state.

The state legislature voted in favour of equal marriage rights for gay couples in February.