Scotland Yard Commissioner Sir Paul Stephenson has asked an independent watchdog to take a second look at its inquiry into taxi driver rapist John Worboys.
The senior officer said he was deeply concerned by an apparently new allegation that officers laughed at one victim of the serial sex attacker.
He has instructed colleagues to speak to those behind an Independent Police Complaints Commission (IPCC) report. They will ask whether the watchdog knew of the claims and if they took them into account as five officers were disciplined.
Speaking at New Scotland Yard, Sir Paul said reports of the woman's comments gave him fresh concerns.
He said: "Irrespective of the circumstances of this particular case, I have got to say I would take the dimmest view of any officer who laughed at a victim. Particularly a victim who reported such a serious crime as this.
"I have asked officers to contact the IPCC once more so that they are fully satisfied that their inquiries fully balanced these issues. I think that is the proper thing to do."
The laughter claim was made in comments by a 21-year-old woman who was assaulted by Worboys as he drove her from central London to Eltham in July 2007.
The woman, a student at the University of Greenwich at the time of the attack, said officers patronised her and left her feeling like a criminal.
She said: "I just felt they weren't taking me seriously at all. When I told them about my injuries they just laughed and said I must have just fallen over."
The move could open the door for a police constable who interviewed the woman to face harsher punishment than the written warning he was given. The second officer, also a constable, has since retired and is no longer subject to police disciplinary procedures.
