It's not quite as gruesome as the drowning in molten gold he metes out to an unlucky character in A Game of Thrones, but George RR Martin has nonetheless threatened to "mount the head on a spike" of the unfortunate Amazon employee who shipped copies of his heavily embargoed new novel, A Dance with Dragons, early.

Out on 12 July, the 1,000-page novel, the fifth in Martin's Song of Ice and Fire series, has been anticipated by fans for the last six years and is subject to a strict embargo by its publisher. Despite this, around 180 copies have been sent to readers by Amazon in Germany, according to Martin and his publisher, and spoilers have begun springing up across the internet.

"I am not happy about this. My publishers are furious. If we find out who is responsible, we will mount his head on a spike," wrote Martin on his blog. He described the situation as "a mess all around" and revealed that other retailers were now "making noises about releasing their own stocks early, using the Amazon error as a justification".

"I know that the 180 readers who got advance copies are happy about this, but I assure you, my publishers are not. And thousands of other readers are now getting spoiled, most quite inadvertently and unwillingly, as they stumble over the spoilers cropping up everywhere on the internet ... Most of those 'lucky' 180 are keeping mum, to be sure, but there are always a few jerkwads in any group, and those are the ones who cannot keep their mouths shut," said Martin. "All I can say is, pfui."

Many new readers have been drawn to A Song of Ice and Fire since the launch of HBO's television adaptation earlier this year, starring Sean Bean. Jane Johnson, who publishes Martin's books in the UK at the HarperCollins imprint Voyager, said she had printed more than 1m copies of the author's books over the last year, with the print run for A Dance with Dragons running into a "phenomenal" six figures.

Voyager appealed to Martin's readers to avoid websites that cover the author's work for the next week "to ensure that the latest instalment of this epic story is not spoiled for you", promising "some stunning surprises" in A Dance with Dragons.

"There are always things to surprise and shock in all of George's books: that's the beauty of George as an author – nobody is safe," said Johnson. "That's the reason we bought him in the first place – the scene where Bran is thrown from the tower is such a shocking one and we were completely mesmerised by that, by a fantasy author who dared to do such a thing … Obviously everyone wants to get the jump on their rivals by selling copies first and fans are desperate to get their hands on it [but] we just want everyone to have a fair chance."