“When I was 21, at my 21st fancy dress party, I wore a Nazi uniform,” he told reporters on Thursday.

“I’m deeply ashamed of what I did and I’m truly sorry for the hurt and the pain this will cause right across our state and particularly to members of the Jewish community, Holocaust survivors, veterans and their families.”

The 40-year-old said the admission was prompted by a colleague coming to him in recent days about the costume. It is understood rumours of a photo had been circulating around Sydney political circles before the premier went public.

“When it was raised with me – this difficult truth of a grave and terrible mistake that I made at my 21st birthday party – to be told by someone else, I felt it was very important that it came from me,” he said.

Repeatedly pausing as he answered journalists’ questions, the Liberal leader denied he was anti-Semitic.

“At that age in my life, I just did not understand the gravity of what the uniform meant,” Perrottet said.

“It was just a naive thing to do – that was a terrible mistake from a 21-year-old who just had no depth or appreciation.”

His parents, who were at the party, raised the issue of the costume with him the next day, he said.

The incident occurred in 2003, two years before Prince Harry was infamously photographed in a Nazi outfit that included a swastika armband.

Mr Perrottet denied his admission on Thursday was prompted by a threat that a photo of him wearing the costume would be released.

Asked if a photo of the costume existed, he said: “I’m not aware of that.”

The father of seven has been premier since October 2021 and is 72 days from going to the polls for the first time as leader on March 24.

The NSW Jewish Board of Deputies said the premier had personally conveyed his “deep and sincere regret about his poor choice of costume as a young man” in a conversation shortly before Thursday’s press conference.

“This incident, no matter how old, is a reminder of the need to continually educate all Australians – and particularly our youth – about the abhorrent nature of the Nazi regime and the evil perpetrated in service of the Nazi ideology,” president David Ossip and chief executive Darren Bark said in a joint statement.

They said the premier had been a staunch supporter and friend of the NSW Jewish community, particularly when as treasurer he secured funding for the Sydney Jewish Museum.

“Nazi symbolism is not to be taken lightly and dressing as a Nazi is not a joke,” Mr Ossip and Mr Banks said.

“The premier has acknowledged this, recognising that wearing the costume was offensive and will distress many in our community. We hope that this unfortunate incident will serve as a lesson to all.”

RSL NSW said in a Twitter post that it noted the premier’s apology, adding the veteran-support charity was conscious of the harm caused by the Nazi regime to millions, including to Australian service personnel and their families.