A Christian web designer is to appeal after a US court said she could not refuse to create websites or content for same-sex weddings.

Lorie Smith was seeking an exemption to the Colorado Anti-Discrimination Act on grounds of her religious beliefs. 

But in a ruling on Monday, the court said Smith and her web design company, 303 Creative, must provide the same services in relation to LGBT weddings as they do for traditional ones.

It means that Smith could be prosecuted under the Act if she refuses to create websites that celebrate same-sex weddings.

Judge Mary Beck Briscoe said, "Colorado has a compelling interest in protecting both the dignity interests of members of marginalized groups and their material interests in accessing the commercial marketplace."

303 Creative had wanted to put a notice on its website stating that it would not make websites for same-sex weddings, but the court concluded this would amount to "unlawful discrimination", and was not covered by free speech protections in the First Amendment. 

Smith's legal counsel, John Bursch of the Alliance Defending Freedom, said Smith plans to appeal the ruling. 

"The government should never force creative professionals to promote a message or cause with which they disagree. That is quintessential free speech and artistic freedom," Bursch said.

"Lorie is happy to design websites for all people; she simply objects to being forced to pour her heart, imagination, and talents into messages that violate her conscience."