The Swedish band, due to perform The Worrying Kind in the second heat in Gothenburg on Saturday night, have been told by composer Barry Mason that their song is plagiarised from his seventies hit Love Grows (Where my Rosemary Goes), Swedish tabloid Expressen reports.
Mason's song was originally recorded by Edison Lighthouse, and was UK number one for five weeks in 1970. Mason is a well known songwriter whose work has been performed by Tom Jones, Rod Stewart, Elvis Presley and Barbara Streisand.
"If they'd asked I would have given them permission to borrow parts of the song," he said.
If The Ark wins Melodifestivalen, as many have predicted, it will go on to represent Sweden at the Eurovision Song Contest in Helsinki in May.
Ola Salo, lead singer of The Ark, denied that he had copied Mason's song.
"You can usually find songs in Melodifestivalen that sound nearly the same as other songs," he told Expressen.
"As long as there's something newly created in it, it doesn't matter if it's reminiscent of something else," he added.
Martin Österdahl, project leader of Melodifestivalen, told Expressen that he would not take any action unless he received a formal complaint from Mason's record company.