Marilyn Monroe’s “Happy Birthday Mr. President” is among the most famous and suggestive songs ever sung in public.

Now it appears that her sexy rendition of “Happy Birthday Mr. President” had a more mundane reason why it was so provocative – she was out of breath.

Marilyn Monroe apparently got lost while heading toward the stage at Madison Square Garden in New York in 1962 to sing for John F. Kennedy.

As the actress ran around trying to find the right door she ran out of breath – so was unable to sing properly – as she got to the stage so late.

Instead Marilyn Monroe’s version of “Happy Birthday” came across as rather more sensual than she may have intended, earning it an unintended place in history.

Marilyn Monroe apparently got lost while heading toward the stage at Madison Square Garden in New York in 1962 to sing for John F. Kennedy

The surprising disclosure was made by American actress Joan Copeland, 89, who was there on the night.

Joan Copeland is the younger sister of playwright Arthur Miller, Marilyn Monroe’s former husband.

She said that Marilyn Monroe was so late to go on stage that night that she missed her cue.

Watching her “running around trying to find a door”, Joan Copeland noticed that Marilyn Monroe was out of breath and anxious.

Rat Pack singer Peter Lawford announced her entrance but when she did not emerge he joked that she was the “late Marilyn Monroe”.

Upon finally coming on stage, Marilyn Monroe stunned the audience with her backless flesh coloured gown with 2,500 rhinestones sewn into it.

The Jean Louis dress was apparently so tight that it had to be sewn on to her naked body – yet it was the rendition of the song which caused all the attention.

Marilyn Monroe performance was given an extra charge because of rumors of an affair between herself and JFK, which have been picked over by fans of both ever since.

When Marilyn Monroe had finished singing JFK came on stage and, in reference to the highly suggestive performance, said: “I can now retire from politics after having had Happy Birthday sung to me in such a sweet, wholesome way.”

The event, which was not attended by Jackie Kennedy, took place on May 19 1962 to mark his 45th birthday, which fell ten days later.

The following November JFK was gunned down by Lee Harvey Oswald.

The dress worn by Marilyn Monroe, who died in 1962 at the age of 36, has since become as famous as the song itself.

In 1999 it went up for auction in New York and sold for $1.26 million, a record for a gown worn by her at the time.

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