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A Labour activist was sexually assaulted at a party event and told not to report his abuser, he has claimed.

He said his attacker, who now advises Labour MPs, “forced his hands down my trousers, feeling my flaccid penis and clutching my face to try and kiss me.”

When the alleged victim, who was 20 at the time, reported the abuse at a Labour Students disco in 2013, a senior member said that “it would not be very wise to pursue my case.”

And he recalled that others in Labour told him that “I should be careful – he’s a clever guy, he’ll be out to get you.”

Writing on the Mirror Online, the man – who wanted to remain anonymous – said Labour Students was “a competitive, sometimes nasty and often alcohol-fuelled environment.

“It’s a breeding ground for hushed-up sexual harassment,” he added.

The abuse followed a Burns Night Supper in Edinburgh attended by more than 100 people, including then-Scottish Labour leader Johann Lamont and other high-profile party figures.

Many of the guests went to the Edinburgh Students’ Union for a party, where the alleged victim found himself dancing alongside a man he already knew.

“Then,” he said, “out of the blue and without saying a single word,” the man forced himself on the activist.

“I was stunned. I didn’t know who to turn to and, the room being so dark, it wouldn’t have been obvious what had just happened.

“My only priority was to get away from him.

“I pushed him away and left that part of the dance floor. I spent the rest of the night on the edge of the event.”

He said that “the next day, I had to sit alongside my attacker in party workshops.”

The situation only spiralled from there, he said.

“I had hardly processed what he did to me when he turned nasty.

“Within a day, he began making vicious comments and bad-mouthing me. He would say I was a bad person, I couldn’t be trusted or I shouldn’t be the candidate for something.

“It would be the start of a more than a year of attacks to undermine me within the party.”

The student, barely out of his teens, said his attacker reacted with a concerted campaign to ensure he never had to face any consequences for the abuse.

“It didn’t occur to me at the time, but he was very clever. Much cleverer than me.

“He sought to immediately put me on the back foot, scaring me before I would have the confidence to report him,” he said.

The alleged victim said that “at the time, I could hardly process what had happened for myself, let alone work out how to explain it to others.”

Weeks later, he told a Labour Students member who was more senior than him.

The figure, who “wasn’t qualified to assess what I was describing,” tried to dissuade him for reporting the abuse.

“I felt powerless. There was no indication of how to report the assault, no support and no sense that if I pursued him, it wouldn’t damage my career.”

And with others in the party warning him to be “careful,” he stopped pursuing justice.

“I really wanted to get somewhere in Labour, so I’m ashamed to say I cowered away,” he said.

“Reporting it to police didn’t even cross my mind. It didn’t occur to me that such behaviour would be taken seriously as a crime.

“More than 12 months later, I was left alone with him for the first time since the attack. I had been drinking with friends in a bar popular with MPs and their staff.

“I didn’t know he would be there beforehand, and I was very nervous to be left alone with him.

“He smashed a Prosecco glass on the table, swore he would ruin me, and stormed out. It was so absurd, I realised it was the first time I wasn’t afraid of him.”

The alleged victim said his attacker is not an MP, but “he is working in parliament, sitting at the party’s top table advising MPs on the policies a Labour government should pursue.

“The thought that he could abuse more people, and amass more power, terrifies me.”

He was driven to go public now “after seeing the shocking stories from Westminster and the likes of activist Bex Bailey, who says the party hushed up her rape report”.

“What pushed me to speak out was the pattern of behaviour he used to stop me talking about it, or if I did, to stop me being believed.”

He said that the latest reports of harassment, “and the serious words from the likes of Jeremy Corbyn,” had given him the confidence he needed to open up about the allegations.

Four years on, he said, he could “finally report my abuser.”

A Labour spokeswoman told the Mirror Online that the party “takes all complaints of sexual harassment, abuse and discrimination extremely seriously and any evidence that is presented to us of such misconduct will always be thoroughly investigated.

“We ask that anyone with a complaint comes forward so that a proper investigation can be carried out and when evidence of misconduct comes to light, all appropriate disciplinary action will be taken in line with the Party’s rule book and procedures.”

She added: “Any individual with allegations of a criminal nature should contact the police and we urge them to do so.”

Shadow Secretary of State for Women and Equalities Dawn Butler has been asked for comment. We’ll add it here if she responds.