It was all going so predictably well. Coldplay's new album went straight to No 1 on Sunday, selling 300,000 copies in three days; concerts sold out; that iTunes ad was everywhere. Even their notoriously sniffy critics in the music press seemed, with the odd exception, unusually muted.

Then, things took a sudden turn for the worse – with a plagiarism row. Yesterday, the band was forced to issue a categorical denial of allegations that they copied the title track to their new record, Viva La Vida Or Death and All His Friends from a little-known US group, Creaky Boards.

In a video posted on the video-sharing website YouTube, Andrew Hoepfner, Creaky Boards' singer and songwriter, claimed that the melody of Coldplay's song, "Viva La Vida", is pinched from a track he wrote last year called, ironically, "The Songs I Didn't Write".

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He blamed Chris Martin for the alleged artistic theft, saying that Coldplay's frontman attended a Creaky Boards concert in New York last year. "We were flattered when we thought we saw Chris Martin in the crowd," said Mr Hoepfner. "He seemed pretty into it... Maybe TOO into it?"

The clip, which was first posted on Sunday, rapidly went viral. By last night, it had been watched by nearly 300,000 people, many thousands of whom had typed comments remarking upon the various similarities between the two tracks. In an industry where even small chord sequences can become subject of costly copyright disputes, allegations of plagiarism are as potentially damaging to a musician's finances as they are to their reputation.

Little surprise, then, that Coldplay responded with a vigorous denial. "We totally refute their claims, and there are two facts that make it easy to disprove them," said the band's spokesman Murray Chalmers. "First, on the night in October when the band say Chris Martin was watching them, he was actually working at the Air Studio in London, and we can prove that. Second, even if he had been at the gig, "Viva la Vida" was written and demoed seven months before the night in question, so it couldn't possibly have been copied."

Sources close to the band said they were unlikely to pursue legal action against Creaky Boards, since it would "look bad" to start a David versus Goliath lawsuit against a group of young musicians. They are, however, pushing for them to publicly withdraw the allegations of plagiarism.

The two tracks have different lyrics, say the Coldplay camp. Although certain elements of their melody sound remarkably similar, the band say this is due to simple coincidence rather than a case of artistic theft.

Either way, the trite nature of Mr Hoepfner's video clip has succeeded in gaining a new following for his band, and was driving traffic to their MySpace page. The YouTube video concludes: "I wish Coldplay the best of luck. If they ever want to collaborate, I've got some microphones we could use in my bedroom."