The sexual misconduct scandal at Oxfam deepened on Monday night as the charity's former head of safeguarding revealed teenage volunteers at UK shops had been abused and overseas staff had traded aid for sex.

In some of the most explosive allegations yet against the charity, Helen Evans accused her bosses of ignoring her evidence and her pleas for more resources, forcing her to quit in despair.

Ms Evans said that staff had been accused of rape and that sexual abuse by shop managers in UK stores against young volunteers was covered up.

Ten per cent of staff in some countries had been sexually assaulted by colleagues or witnessed abuse, she added.

Her allegations emerged just hours after Penny Lawrence, the charity's deputy chief executive, quit over the scandal and the Government announced that it would be launching a unit to investigate sex abuse in the aid sector.