The raven-haired beauty got very emotional as she expressed her feelings for the 54-year-old music mogul - who turns 55 on October 7 - while making a speech at the bash at west London restaurant Scalini.
Singer Lucy Kay told the Daily Mirror newspaper: ''Lauren's speech was emotional, she was crying and saying how much she loved him.''
The 36-year-old beauty - who has seven-month-old son Eric with Simon - wasn't the only one to make a speech at the party as the 'X Factor' boss' fellow 'Britain's Got Talent' judge David Walliams also took to the microphone and continued their bromance by pretending to be Simon's boyfriend.
A party source added to the publication: ''There was a few speeches and David Walliams did one which was very funny.
''It was what David's humour is always like with Simon, he was playing out the boyfriend role.''
Several other stars were also in attendance at the bash including Simon's fellow 'X Factor' judge Cheryl Fernandez-Versini and her new husband Jean-Bernard.
The birthday boy was presented with a comical cake which featured a figurine of his baby Eric sitting in the middle of a calorific 'X Factor' judging panel alongside Simon's pet pooches Squiddly and Diddly.
A message in front of the tasty table read: ''New judging line-up.''
Guests were treated to a slap-up meal of Simon's favourite cuisine, spaghetti bolognaise, at the bash and some partied until the early hours.