Senior Scotland Yard officers admitted tonight they are deeply concerned about the rising number of gang rapes.
New figures revealed there were 93 gang sex attacks in 2008-9, compared with 71 in 2003-2004.
Meanwhile the age of victims has fallen with 64 per cent aged 19 or younger in the last financial year compared with 48 per cent in 1998-9.
Police define a gang rape, which they term multi-perpetrator rape, as being a sex attack involving three or more people.
Commander Simon Foy, who leads the Met's homicide and serious crime command, said there is no doubt the "abhorrent" crime is under-reported.
He said: "This is a phenomenon we are all concerned about. We know this is an area that is under-reported.
"There is a substantial amount of this type of offending going on which we do not necessarily know much about.
"The numbers we do have are relatively small. That makes it difficult to understand the trends and behaviours that are going on.
"There is no doubt that the number of multiple offender gang rape offences is going up and we can say there is an increasing number of offences with four or more suspects.
"The greatest proportion of victims of this type of offence are under 19 and there is a very significant number under 15.
"Young people are particularly at risk of this type of offending."
There have been a series of high-profile convictions of teenagers for gang rapes in the capital over the past year.
Two men who assaulted a girl aged 16 and doused her in caustic soda, disfiguring her for life, had their sentences increased on appeal.
In another case a 14-year-old girl was repeatedly raped "as punishment" by nine members of a Hackney gang because she had "insulted" their leader.
A meeting of the Metropolitan Police Authority (MPA), the Met's board of governors, heard levels of gang rape are linked to overall youth violence.
Boroughs with the highest numbers of gang rapes include Lambeth, Croydon, Newham, Southwark, Westminster and Hackney.
The Met has commissioned research from Dr Miranda Horvath, a lecturer in forensic psychology at the University of Surrey.
She is focusing on the "cultural context" of gang rape and speaking to officers from forces across Britain and the United States.
Jennette Arnold, who represents Hackney, Islington and Waltham Forest on the MPA, said some offenders are from cultural backgrounds where rape is more common.
She said the crime is seen by some as a "weapon of war" and more work needs to be done to get into the minds of culprits.
Mrs Arnold said: "It has got to be regretted that the increase in black victims has doubled."
Chris Boothman, another member of the MPA, said he remembers gang rapes taking place when he was a teenager growing up in London.
He said it is the responsibility of other agencies to intervene among young men who may be involved in, or are aware of, gang rape.
Mr Boothman said: "There is a massive piece of education to be done in schools and youth clubs in terms of unpicking an area of activity that groups of young men believe is acceptable."
The meeting heard workers trained to work with young people are based at centres for sex attack victims, known as Havens.
Officials based in Whitechapel and Paddington have also visited secondary schools and youth groups across London to dispel myths around sexual violence.
Cmdr Foy said: "What I do not understand is what motivates people to commit this particular type of offence in these particular circumstances.
"It is often clear why someone would carry a gun or knife. What is the propensity to commit these criminal offences and how does it manifest itself?"