When art conservators joined hands to restore two rare portraits of Shakespeare they thought they were removing paint daubed on the canvases more than 100 years after the Bard's death to reveal "authentic" portraits beneath.

Now it has emerged they were, in fact, wiping away priceless insights into the changing appearance of Britain's greatest playwright.

The images which had been superimposed on both paintings had actually been painted in Shakespeare's own lifetime, the Art Newspaper will reveal next week, and showed how he looked as he aged. The so-called "restoration" could now go down in art history as one of the biggest blunders on record.

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A newly discovered picture of Shakespeare called the Cobbe portrait (painted when he was still living) and another version called the Folger portrait were both irreversibly "cleaned up" in this way.

New research has revealed both portraits were probably altered during Shakespeare's lifetime, or within a decade or so of his death in 1616, while his friends and associates were still alive. In the Cobbe portrait, the sitter was given a bouffant hairstyle, whereas in the Folger portrait, his hair at the front was replaced by a bald forehead.

But why the changes? The Cobbe work is believed to have been painted for the Earl of Southampton. The Shakespeare expert Stanley Wells suggests the Bard had dedicated his erotic sonnets to him. It is possible the Earl may have wanted a more flattering image.

The Folger portrait, on the other hand, may have been altered to reflect Shakespeare's appearance at the time of his death, six years after the original painting. The original represented Shakespeare aged 46.

Rupert Featherstone, director of the Hamilton Kerr Institute in Cambridge, which undertook technical investigations into the Cobbe portrait, admitted that in hindsight, it was unfortunate conservators had removed the overpaint. "We can no longer peer down a microscope to look at the physical evidence of the overpaint," he said. When the overpaint was removed from the two portraits, in 1988 and 2002, it was not thought that either depicted Shakespeare. Some critics doubted that the Bard sat for either portrait.

The Cobbe portrait was restored in 2002 as part of ongoing conservation work of the Cobbe family's pictures. It was then thought that it depicted an unknown sitter by an anonymous artist. The conservation work was undertaken by Mr Cobbe, who is a professional restorer. Research now shows the Cobbe painting is an original portrait completed in Shakespeare's lifetime, and that the Folger picture is an early copy, painted in 1610 when the playwright was still alive.

Mr Cobbe now believes his portrait may have had the hair repainted as early as a few months after the original work had been completed in 1610.

The Folger painting, which was conserved in 1988, is in the Folger Shakespeare Library in Washington. It was acquired in 1932 as an image of Shakespeare, but later downgraded to an anonymous portrait.