Google has become embroiled in a row about gay marriage, after opponents of same-sex unions in California used the search engine's advertising network to post messages on popular websites against the owner's wishes.

A number of American websites, including the widely read TechCrunch and Create Digital Music blogs, featured adverts backing Proposition 8, which would change California’s constitution to define marriage as the union of a man and a woman.

The adverts appeared on the sites a day before the crucial vote - which Californians will take at the same time as the presidential ballot - and have caused uproar among site owners and readers alike.

Proposition 8 marks the first time voters have been asked to ban same-sex unions retroactively. If passed, it would overrule the state Supreme Court decision in May that said preventing gays from marrying was unlawful discrimination. Since that ruling more than 16,000 gay couples have wed in California.

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Peter Kirn, from Create Digital Music, told Times Online: “Not only was this ad was something I disagree with – it could have said 'Vote For McCain' even though I’m an Obama supporter – but it is also an ad that could be seen as offensive to some of my readers."

He added: “This an extraordinary breach of trust we’ve put into Google ads.”

Mr Kirn has tried to use Google’s own software to remove the adverts from his site and has contacted the company about the issue. When the adverts remained on the site for several hours despite Mr Kirn’s best efforts to remove them, he said he was forced to remove all Google adverts from his site.

Popular technology websites, followed closely in Silicon Valley and the rest of California, seem to have been the main targets for the adverts.

“Denying a fundamental right to a person - such as marriage - is hateful and backwards,” wrote Michael Arrington on TechCrunch. “Google doesn’t have any obligation to run ads like these, and I believe they would be correct in banning.”

A Google spokesperson said: “Google allows ads that advocate for particular political position, regardless of the views that they represent. We’re currently allowing ads advocating both for and against Proposition 8.” Google insisted that there were controls in place to avoid adverts appearing that may offend the website owner.

The row may embarrass Eric Schmidt, Google's chief executive, who appeared on a “NoOnProp8” advert that appeared in a major Californian newspaper just days ago.

More than $65 million has poured in to both sides of the argument from across the United States and the world, making the initiative the most expensive election question this year, apart from the race for the White House. Much of the money for the yes campaign has come from Christian organisations, while Hollywood studios have contributed to the no camp.

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