A former parliamentary intern has told the BBC that he was sexually assaulted by a former MP in 2012.
James Greenhalgh, who didn't know the MP, said he felt violated.
He said when he tried to report the assault a couple of months later, he was told by the MP's party that he couldn't make a complaint anonymously so did not proceed.
The party concerned said it took "any allegation of this nature extremely seriously".
Mr Greenhalgh told the BBC he was approached outside one of the bars in the House of Commons by the man who put his arm around him and then went on to assault him.
"He literally put his arm around me, very close, stinking of alcohol I remember and pointing out different things on the canvas [painting]," he said.
"I was interested to hear what he had to say, but I was thinking, 'This is very, this is very touchy-feely here - what's he doing?'
"And suddenly his arm slipped down towards my buttocks, and he had a good feel round there and went a bit further in between my legs.
"It wasn't very pleasant at all. I just didn't know what to do, I didn't know what to do at all."
The disclosure follows a range of recent allegations in Westminster, including claims of a lack of support for those making complaints.
Earlier, at Prime Minister's Questions, Theresa May said any allegations about serious sexual abuse in Parliament should go to the police, as she promised a new "independent" process to handle complaints.
The PM said she was "deeply concerned" by recent reports about alleged harassment and abuse at Westminster.
Labour's Jeremy Corbyn said trade unions should be involved to support staff.
Mr Corbyn said he was happy to meet the PM to discuss it, with a meeting scheduled for Monday evening.
The PM said a "common, transparent independent grievance procedure" for all those who work in Parliament was needed and that it "cannot be right" for policies to vary between parties.
A dedicated support team should be available to all staff, she said, and it should recommend all criminal allegations be reported to the police.