The British chief prosecutor of the International Criminal Court allegedly sexually assaulted a female colleague in a hotel room in Africa just hours before making an impassioned speech, via video-link, to the United Nations.

The Mail on Sunday can reveal that Karim Khan KC, the barrister bringing a controversial war crimes case against Israel, is alleged to have coerced the woman into having non-consensual sex with him during an official trip to Chad, where he met victims of sexual violence.

The alleged incident is said to have happened at the Radisson Blu hotel in N'Djamena, Chad's capital, when Mr Khan is claimed to have asked the female lawyer to come to his luxury suite, where he held meetings, to review a speech he was due to make remotely to the UN Security Council.

It is understood the woman, a lawyer in her 30s, claims she told him multiple times she wanted to leave the room – but that he took her mobile phones and repeatedly pulled her by the arm towards his bed after she stood still.

Later that day he briefed the UN about war crimes, including rape, being committed in neighbouring Darfur and about the traumatised refugees he had recently met in two camps on the Sudan border.