NEW DELHI (AFP) - India's leading investigative news magazine was embroiled in scandal on Thursday after its editor admitted sexually assaulting a colleague and said he would step aside for six months as "penance".

Tarun Tejpal, founder and editor of Tehelka magazine, wrote to the managing editor of the publication admitting "misconduct" which he said he must atone for by agreeing to stop working for six months.

"A bad lapse of judgement, an awful misreading of the situation, have led to an unfortunate incident that rails against all we believe in and fight for," Tejpal wrote in an email sent to staff at the magazine on Wednesday.

Women's right groups criticised his response as insufficient and the revelations led news bulletins on cable networks.

"Sexual harassment is a crime... Tarun Tejpal & Tehelka must act acc(ording) to law, face consequences," tweeted Kavita Krishnan, secretary of the All India Progressive Women's Association.

India's media and rights groups are now more vocal than ever after a fatal gangrape of a student in December last year galvanised public disgust against rising sexual violence and the country's gender imbalance.

The government passed a law toughening sentences for sex offenders in March, containing new penalities for a rape, stalking and groping.

Authorities would only investigate the incident at Tehelka if the victim files a police case.

The weekly magazine has made its name with a series of major national scoops and sting operations revealing cricket match-fixing, the involvement of politicians in religious riots in 2002 and bribe-taking.

The scandal broke just after police in New Delhi launched a probe into allegations of sexual harassment against an unnamed retired Supreme Court who was implicated by a young female lawyer.

India's opposition prime ministerial candidate Narendra Modi is also in the eye of a storm following revelations in the media that one of his top aides ordered police surveillance on a woman at the behest of his "master".