Anneka Sutcliffe said she tried to visit her friend at Thameside Prison who is being held for climbing on top of a train at Canning Town station during Extinction Rebellion protests.
Ms Sutcliffe claimed she approached the security checkpoint with the correct paperwork before she was searched.
But because of the "missing" underwear, she was told she could not enter the building. Speaking to the Standard today, she said her “breasts have never been so angry”.
She said: “I had carefully read the HMP Thameside website, made sure I had the right paperwork, dressed accordingly, wearing no sharp jewellery etc.
“I got there on time but me and my friend Sally were refused entry at the security checkpoint, where a female guard did the usual airport-style feel-you-up, because she noticed we weren’t wearing bras.”
Thameside Prison said all visitors are required to be “appropriately dressed”, and unless there are medical reasons all women must wear bras for “decency of other visitors” including children.
It also said wearing bras helps detect illicit items and prevents allegation of inappropriate contact by staff.
But Ms Sutcliffe said of her attempted visit on Wednesday: “I did not foresee that my morning, after getting a 4.30am coach to London, would involve attempting to have a logical argument about the literal interpretation of female ‘underwear’ with the prison manager.
“I hardly ever wear a bra. To me, a vest top and pants are my underwear, and that’s my business anyway.
“A man in the queue in front of us said: ‘I’ve got bigger t*** than you and I’m not wearing a bra!’"
The climate change campaigner, from Bristol, said she was not dressed provocatively and was “angry, sad and shocked” that she couldn’t visit her incarcerated friend who she claimed is only granted one visit a month.
She was attempting to visit ex-Buddist teacher Mark Overton, 36, who she said had been arrested more than 10 times to “highlight the climate emergency” and also glued his hand to a plane at City airport, as well as spraying red liquid on the Treasury building.
Ms Sutcliffe said Mr Overton is one of the only protestors to still be behind bars after he did not apply for bail. He is on remand with his trial due to start in December.
“I was really looking forward to visiting him," she added. "It took him a while to get registered as a vegan and he spends his time reading a lot of books and meditating.
“He is trying to counsel his cellmate through drug and alcohol issues and is seeing prison as a real time of servitude - trying to help other prisoners – which is wonderful.
“He didn’t apply for bail as he was fully prepared to go to prison for his actions. It highlights the injustice of being in prison for non-violent protest when our government is guilty is ecocide.”
Craig Thomson, Serco's prison director at the jail, said: “All women are required to wear underwear including a bra when visiting the prison, not just in the interests of other visitors, particularly children, but also to help our staff who are searching for contraband.
“Anyone who is abusive or intimidating towards our staff will be refused entry to the prison.”