A Labour activist has said she was raped at a party event and that a senior Labour official discouraged her from reporting the attack.
Bex Bailey said she was told reporting the 2011 incident could "damage" her and that she was given no advice on what she should do next.
She told the BBC she had waived her anonymity to urge changes to the way such cases are handled.
Labour said it had launched an independent investigation.
This will look at "claims that a party employee acted improperly over these 2011 allegations", the party said.
Labour leader Jeremy Corbyn said Ms Bailey had shown "incredible bravery" in speaking out and said she had his "full support and solidarity".
The issue of sexual abuse within politics and the parties' response has come under the microscope after recent allegations about sexual harassment in Parliament.
Labour has said "robust procedures" both "inside as well as outside Parliament" are needed and Jeremy Corbyn has written to members urging anyone with a complaint to come forward using "confidential party procedures".
Ms Bailey, who is calling for an independent body free from political "bias", said there was now a recognition that "it's a problem in every party at every level".
The 25-year-old is a former member of Labour's ruling National Executive Committee.
In an interview with PM on BBC Radio 4, she said she had been 19 when she was raped by someone senior to her within the Labour Party who was not an MP.
She said she had "tried to pretend it hadn't happened" and did not report the attack to the police at the time.
"I was scared, I felt ashamed, I know that the Labour Party, like any family, loves a good gossip - and I didn't want people to know and I also was worried that I wouldn't be believed if I did," she said.
Two years later, she did confide in a party official.
"It took me a while to summon up the courage to tell anyone in the party," she said.
"But when I did, I told a senior member of staff, who told me... or it was suggested to me that I not report it, I was told that if I did it might damage me - and that might be their genuine view, it might be that that was the case in which case that shows that we have a serious problem in politics with this issue anyway."
Ms Bailey said she was not given good advice and was "not signposted to anyone else that could", and there seemed to be no procedure to report the incident.
"I don't think I was even given a cup of tea at the time," she said.
"It was quite a horrible experience and this is why I've been fighting so hard for changes to the way that we do this."
She is calling for an independent agency, like a charity, so allegations are dealt with free from "political bias" and complainants do not feel they will be "penalised".
This body should provide advice on taking the matter to the police or, anonymously, to the party, she said.
Labour's process relies on people reporting an attack to someone within the party - who is "inclined to be loyal to the Labour Party", she said.
"It's important that we need to make sure that this results in actual change in our parties as well as in Parliament, rather than letting it all blow over," she said.
Ms Bailey said she had seen "a lot of brave women" speak out in recent days and weeks, and had chosen to do so to secure the changes she has campaigned for.
"I just really hope that all the horrible things that we're seeing will at least result in some sort of change in our parties as well as in Parliament."
A Labour Party spokesman said: "The Labour Party takes these allegations extremely seriously. It takes great courage for victims of rape to come forward - and all support must and will be made available to them.
"We would strongly recommend that the police investigate the allegations of criminal actions that Bex Bailey has made.
"Labour will also launch an independent investigation into claims that a party employee acted improperly over these 2011 allegations."
Mr Corbyn added: "There will be no tolerance in the Labour Party for sexism, harassment or abuse.
"Whatever it takes, we are absolutely committed to rooting it out."
Ed Miliband, who was Labour leader in 2011, said he was "shocked" by Ms Bailey's "horrific allegations".
He added: "She is showing great bravery and courage in speaking out. Victims must be supported when they come forward. These allegations must be properly investigated by the police and the Labour Party."
Meanwhile, the BBC has seen a list, thought to have been compiled by staff and researchers at Westminster, detailing a range of mostly unproven allegations about 40 Conservative MPs and ministers.
Among the claims are a number of serious allegations of inappropriate behaviour with junior members of staff, the use of prostitutes and affairs between MPs.
BBC political editor Laura Kuenssberg described the list as "both a mixture of unsavoury allegations, reports of well-known relationships, and some claims that are furiously denied".