Treatment of female staff at the Presidents Club dinner "fell short of what would be expected in the 21st century", the Charity Commission has said.
In a report published on Friday, the Commission said the Presidents Club Charitable Trust did not look after hostesses working at the event, while at the same time taking "careful steps" to protect the privacy of male guests.
Following reports earlier this year that staff working at the event had been groped by attendees, the charity's trustees announced that the men-only dinner, which has been held annually since 1985, would no longer take place.
The Charity Commission's investigation found that the dress code for 140 female staff working at the dinner in January and the instructions for how they should look were not "acceptable in a charitable environment".
Staff were provided with a dress to wear, told to do their make-up and hair "as if going to a smart sexy place” and to bring “smart sexy shoes with heels”.
The regulator said trustees did not deal properly with the reputational risk caused by holding "an all-male event with predominantly female event staff who had been issued with dresses purchased by the charity and with instructions on appearance".
Helen Stephenson, chief executive of the Charity Commission, said: “Our report should serve as a warning to others that raising funds for charity does not absolve trustees of their legal duties or moral responsibilities."
She added: "The trustees thought insufficiently about the welfare of the women hired to work at their charity’s event while taking careful steps to protect the privacy of the male guests attending the dinner.
"It is not the Commission’s role to determine whether any of the women working on the night were subjected to harassment or abuse.
"What we can say is that the trustees’ attitude towards their welfare in the name of charity fell short of what would be expected in the 21st century."
A separate report released by the Fundraising Regulator warned other charities who benefit from such events that they should do "due diligence" before they agree to be publicly associated with them.
According to the charity's website the Presidents Club had in the past raised money for charities including Scope, the NSPCC and Great Ormond Street Hospital.
Following the allegations, which were first published in the Financial Times, all three cut ties with the charity and said they would not accept any further donations.
The regulator added that the trustees "had little awareness of the expectations around fundraising" and did not properly supervise the company which organised the event.