Bex Bailey, 25, made the claim amid a wave of allegations of sexual assaults in the UK.
“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,” Baily told BBC radio on Tuesday.
Labour leader Jeremy Corbyn ordered an investigation into the allegation and said her act is an indication of her bravery.
“Bex Bailey has shown incredible bravery by talking publicly about what has happened to her and has my full support and solidarity,” he said in a statement on his Facebook page.
“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.”
Also, a party spokesman said that they would ask police to launch an investigation.
Bailey said she infomred a senior party staff member of the matter in 2013, but the member advised against reporting it.
“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,” said Bailey, a former member of Labour’s National Executive Committee.
The number of abuse allegations being made in the UK has spiked since one of the BBC’s top presenters, Jimmy Savile, was exposed as a serial pedophile after his death in 2011.
According to a report last year by the royal Inspectorate of Constabulary, hundreds of British police officers and security forces have used their power to sexually abuse vulnerable people.