The computer-generated image of the main suspect in Madeleine McCann’s abduction is said to be so vague it resembles an “egg with hair”.

Police have been touring the Algarve asking shopkeepers and local people if they recognise the potential suspect, but have refused to release the image to the public, claiming that it would prejudice a future case.

Simon Russell, 40, who runs a video store and internet café in the resort town, said: “They just showed me this outline which gave me the impression that it had high cheekbones and a long face but other than that, no eyes, no nose, no mouth. It was an egg with hair.”

Mr Russell sketched the outline after being shown it.

There have been claims that the Policia Judiciária, which is leading the investigation, held back from showing the image to Portugal’s main police force until Monday, four days after Madeleine went missing. Yesterday Susan Healy, Madeleine’s grandmother, defended her daughter from accusations that she was negligent.

Kate and Gerry McCann have faced criticism for leaving Madeleine and their 18-month-old son and daughter alone while they dined at a nearby restaurant. Mrs Healy said: “To hear that sort of criticism when you know Kate and Gerry are such caring parents does not help the situation.” Newspaper reports suggest that the McCanns may be questioned under police caution. In an interview last night, Mrs Healy said that she thought mistakes could have been initially made in the police investigation due to “inexperience”.

She said it had been difficult for Mr and Mrs McCann to get across to the Portuguese police what had happened. She added however: “There isn’t much point in debating what went wrong because it doesn’t achieve anything.”

Mrs Healy described her granddaughter as a “diamond — funny and bright with a sense of humour”.

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