Prisoners of "Gypsy, Romany or Traveller" backgrounds are "significantly" over-represented in jails in England and Wales, inspectors have found.
One in 20 inmates - or 5% - told Her Majesty's Inspectorate of Prisons (HMIP) that they considered themselves to be Gypsy, Romany or Traveller in 2012/13, compared to 4% the previous year.
Based on the most recent prison population figures from the Ministry of Justice, 5% would equate to just over 4,200 inmates and is roughly similar to the total number of women in prisons.
Proportions of gypsies are even higher in Secure Training Centres (STCs), which hold young people between 12 and 18 years old, with 12% considering themselves Gypsy, Romany or Traveller. In Medway STC in Kent, this figure was 22%.
Just 58,000 or 0.1% of people identified themselves as "Gypsy or Irish Traveller" in the 2011 census.
In its paper, HMIP said: "Even on the lowest estimates... it is clear that prisoners of Gypsy, Romany and Traveller backgrounds are significantly over-represented in the prison population.
"Our survey findings suggest that the proportion might be as high as 5% - the same proportion as women prisoners - and much higher than this in some establishments, particularly those holding children.
"The reasons for this over-representation lie outside the prison service and more needs to be done to understand and address this."
Gypsy, Romany or Traveller numbers are "strikingly high" in some prisons, HMIP added.
In 2012/13, 12% of prisoners at HMP Elmley, Kent, 11% at HMP Gloucester and 10% at HMP Winchester identified themselves as being Gypsy, Romany or Traveller.
In 2012/13, 5% of 15 to 18-year-olds held in young offender institutions (YOIs) identified themselves as Gypsy, Romany or Traveller, the same proportion as in the adult estate, although in YOI Warren Hill, in Suffolk, it was 12%.
A Ministry of Justice spokeswoman said: " We are committed to ensuring fair and equal treatment for all prisoners, including those with protected characteristics as defined in the Equality Act 2010.
"As part of this, we recently launched a campaign to encourage and support Gypsy, Traveller and Roma prisoners to declare their heritage and address some of the sensitive issues affecting them and their communities. Since the start of this, we have seen a 50% increase in declarations."