One in four areas had a rape conviction rate of less than 5% in 2007
The "postcode lottery" of justice faced by rape victims in England and Wales is getting worse, a report has said.
The Fawcett Society said Ministry of Justice figures showed a growing gap in rape conviction rates between areas.
Women in Cleveland were more than 11 times more likely to see their attackers found guilty than in the county of Dorset, it said.
The Home Office said new measures were being brought in to improve rape case investigation and victim support.
The Fawcett Society's research found that in Cleveland, nearly one in five reported rapes led to a conviction in 2007, but in Dorset fewer than one in 60 women who reported rape saw assailants found guilty.
But while Cleveland showed continued improvement in conviction rates - from 7.75% in 2004, to 13.2% in 2006, and 18.1% in 2007 - researchers found rates had fallen at a "worrying" rate in 16 out of 42 police force areas.
The society said 12 regions had a rape conviction rate of less than 5% in 2007.
Katherine Rake, director of the Fawcett Society, said the figures in most areas revealed women "continued to face a postcode lottery when reporting rape to the police".
"Rape should be treated with the same professionalism as other crimes, with consistency in initial response to victims and investigation across police areas," she said.
"It is a national scandal that thousands of victims have no access to justice, and frequently face a culture of disbelief and delayed responses which may lead to the loss of vital evidence."
CONVICTIONS BY AREA - 2007
N Yorks: 15.4%
Source: The Fawcett Society
She said there was also "patchy provision" of support services for women who have been raped in England and Wales, particularly in rural areas.
"Women deserve support, safety and justice from the criminal justice system and this is not being delivered," she added.
Det Ch Insp Alastair Simpson, of Cleveland Police, said his force had worked closely with the courts, health providers and other voluntary organisations to improve its response to rape cases.
"Cleveland Police takes all allegations of rape seriously. Every allegation is treated as a serious crime inquiry and is investigated thoroughly," he said.
Cheshire's Chief Constable Dave Whatton, who is Britain's most senior officer responsible for rape inquiries, said new guidance had been issued by police and prosecutors to improve investigation.
But he added that there were "many issues" that influenced conviction rates.
"Investigations are often complex, particularly on the issue of consent and often because some victims do not want to follow a prosecution route," he said.
A Home Office spokesman said a number of measures announced in April aimed to improve police investigation of rape and provide further support for victims.
"The measures include helping every police force to ensure that all victims are seen by a specially trained officer within an hour of reporting a crime, better training for officers and a cross-government group to monitor police and CPS [Crown Prosecution Service] performance on rape.
"New guidance for police on investigating and prosecuting rape will be issued shortly and an expert support team will ensure it is delivered consistently around the country."