A Labour MSP has alleged she was sexually assaulted by a senior male colleague at a party as the sexual harassment scandal claimed its first scalp at Holyrood with the resignation of an SNP minister.

Monica Lennon, 36, told a tabloid newspaper she was groped at a social event four years ago in front of a room full of people but she did not pursue a complaint she lodged with her party because she did not think she would be believed.

She said the culprit was a “powerful, influential and well-respected” Scottish Labour figure and witnesses, including someone who was a Labour politician at the time, treated it as a joke.

The allegations by Ms Lennon, who is the most senior elected politician in the UK so far to claim she has been a victim of sexual assault, came shortly after Mark McDonald quit as Nicola Sturgeon’s Childcare Minister.

The Aberdeen Donside MSP refused to disclose details but said he had been guilty of conduct that “might have made others uncomfortable or led them to question my intentions.”

He said he had wrongly believed the behaviour “to have been merely humorous, or attempting to be friendly.” In another hint at the allegations against him, he said he hoped his resignation would prevent “any particular woman” being the subject of “unwarranted” media attention.