Madonna has at times imitated Monroe's bottle blonde looks

Lawrence Nicastro, 73, said he found the grainy poster-sized photo at his home and thought it was a rare picture of Monroe posing as a naked hitchhiker.

He and his wife Phyllis then spent four months researching the photograph.

Mr Nicastro suspected it had been left by a customer at his service station in New York city in 1962.

The couple brought in Monroe expert Chris Harris, who confirmed it was a genuine, exciting find.

Harris scheduled a news conference to unveil the image but allowed some journalists a sneak preview.

They suggested the photographic treasure was in fact a shot of Madonna from her graphic 1992 book, Sex.

It displayed the singer wearing just heels and a handbag, with a cigarette in her mouth.

Harris realised the mistake: "You're right; it's Madonna. If there ever was an embarrassing moment ..."

He still plans to hold the news conference to "face the music".

"Who wins here? Madonna, of course. She really looks like Marilyn Monroe," insisted Harris.

It is not clear where the photograph actually came from, but Harris said he believed someone had slipped the poster into Mr Nicastro's belongings as a practical joke.