LONDON - MAY 19: (FILE PHOTO) A prison guard at HMP (Her Majesty's Prison) Pentonville stands behind a locked gate May 19, 2003 in London.  A new report from the Prison Reform Trust (PRT) says overcrowding in Britain's prisons has been caused by tougher sentencing rather than an increase in crime.  Since 1991, offenders of petty crimes are three times more likely to be imprisoned.  (Photo by Ian Waldie/Getty Images)