A florist in eastern Washington State, in the US, has found herself with a legal battle on her hands after refusing to supply flowers for a gay wedding.

Barronelle Stutzman, who owns Arlene's Flowers, had supplied flowers on previous occasions for openly gay customer Robert Ingersoll.

However, she felt she could not in conscience make the floral arrangements for his upcoming wedding to his partner.

She explained to him at the time that her decision was because she had a moral objection to gay marriage.

According to the Family Research Council, long-time customer Ingersoll respected her opinion and the pair hugged before parting ways.

The organisation said Stutzman was surprised to learn her shop is now being sued for sexual discrimination. She faces a fine of up to $2,000.

Her attorney JD Bristol said the sexual discrimination claim was "nonsense".

"Arlene's Flowers has catered to all patrons, including homosexuals, for many years," he said.

"Arlene's Flowers has had openly gay employees. This is about gay marriage. It's not about a person being gay. She has a conscientious objection to homosexual marriage, not homosexuality. It violates her conscience."

Washington State voted last year to legalise same-sex marriage.