The invite - which told guests not to wear flip-flops or denim - quickly made headlines after astonished colleagues forwarded the email to friends at other City firms.
Now a contrite Ms Gao, a real estate equity researcher, has circulated a second email. It reads: "Hi, fellow Citigroup interns. Just to clear a few things up, now that it is seriously getting out of control, it was suppose(d) to be an internal joke between me and a couple of my guests to my 21st Party.
"I should not have used my Citigroup email account in the first place so I apologise for that. I am sorry if you found the content of the invitation details offensive and I am glad to entertain.
"But please stop now because it is getting really unprofessional and unnecessary."
Oxford-educated Ms Gao's original email dropped such helpful hints as: "The more upperclass you dress, the less likely you shall be denied entry," while "photos will be taken between 10pm and 11pm - therefore you may want to be well groomed".
