Secrets Behind Marilyn Monroe's 'happy birthday Mr President' Moment

It is among the most famous and suggestive songs ever sung in public.

But now it appears that Marilyn Monroe's sexy rendition of 'Happy Birthday Mr President' had a more mundane reason why it was so provocative - she was out of breath.

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

As she 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 Monroe's version of 'Happy Birthday' came across as rather more sensual than she may have intended, earning it an unintended place in history.

Interest in the actress has been reinvigorated by the film 'My week with Marilyn', starring Michelle Williams, which is due out in the UK today.

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

Miss Copeland, the younger sister of playwright Arthur Miller, Monroe's former husband, said that the actress was so late to go on stage that night that she missed her cue.

Watching her 'running around trying to find a door', she noticed that 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, 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.

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Her performance was given an extra charge because of rumours of an affair between herself and Kennedy, which have been picked over by fans of both ever since.

When Monroe had finished singing Kennedy 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'.