Breathlessness behind Marilyn Monroe's most memorable performance

London, Nov 25 : The reason why Marilyn Monroes rendition of Happy Birthday Mr President for John F Kennedy sounded so sexy and provocative was simply because she was out of breath.

The film icon apparently got lost while heading towards the stage at Madison Square Garden in New York in 1962 and as she ran around trying to find the right door she ran out of breath, the Daily Mail reported.

Monroe was unable to sing properly because she got to the stage so late, but her version of Happy Birthday came across as rather more sensual than she may have intended, earning it an unintended place in history

It became the late actress most memorable moment and is among the most famous and suggestive songs ever sung in public.

The interest in the actress has been reinvigorated by the film My week with Marilyn, starring Michelle Williams, but the surprising disclosure was made by American actress Joan Copeland, 89, who was there on the night.

Copeland, the younger sister of playwright Arthur Miller, Monroes 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, Copeland 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 that caused all the attention.