Private contractors exposed w omen prisoners to "intimidation and abuse" by forcing them to travel in escort vans with male inmates.
An unannounced inspection of the largest women's prison in Europe, HMP Holloway, found joint-venture firm Serco Wincanton was transferring female inmates to the jail in "grubby" vehicles with men for long periods without a privacy screen.
Women prisoners faced lengthy journeys as m ale inmates were taken to jail first because reception areas in men's prisons have a fixed cut-off time, HM Inspectorate of Prisons (HMIP) said.
Juliet Lyon, director of the Prison Reform Trust, said: "It's time to call a halt to transporting women, many of whom have been victims of sexual abuse and domestic violence, with men in prison vans where too often they are subject to long delays and intimidation.
"In London and elsewhere, women are regularly dropped off last after long, gruelling journeys from prison to prison because prison escorts know that staff in women's prisons will stay late to receive vulnerable women entering custody while staff in men's prisons shut their doors earlier.
"Overall transport of livestock is better regulated than transportation of prisoners."
Serco Wincanton, which has a fleet of 200 secure vehicles, transfers inmates between prisons, police stations and courts in London and the east of England, including The Royal Courts of Justice and the Old Bailey.
Frances Crook, chief executive of charity the Howard League for Penal Reform, said: "These "sweat boxes" are pretty disgusting. On long journeys people are given plastic bags to pee into, pretty tricky in a moving vehicle for women.
"Women should be moved in separate vehicles and don't need to be in sweat boxes as they do not represent a security risk."
Holloway prison, which recently held former Cabinet minister Chris Huhne's ex-wife Vicky Pryce, has become safer but still has work to do, Chief Inspector of Prisons Nick Hardwick said.
There were few serious violent incidents and the use of illegal drugs was relatively low, Mr Hardwick added.
But he said the jail, which can hold nearly 600 inmates, is a difficult prison to run due to its size and poor design.
The prison did not do enough to help women have positive relationships with their children and families and too few activity places were used, he said.
In his report, Mr Hardwick said: "Women spent long periods in escort vehicles shared with men before arriving at the prison. Some vehicles did not have privacy screening, exposing women to the possibility of intimidation and abuse."
A Government-wide review of all contracts held by Serco, one of the country's biggest private providers of public services, is currently being conducted.
The audit, triggered by revelations that Serco, as well as G4S, had overcharged the Government for criminal-tagging contracts, prompted calls for the Ministry of Justice to abandon its plans to privatise the probation and prison service.
Michael Spurr, chief executive of the National Offender Management Service (NOMS), said: " Staff at Holloway deal with a really challenging population, including many vulnerable women with complex needs, and I am pleased that this inspection acknowledges the excellent work being done by the governor and her staff.
"The progress made reflects their commitment, care and dedication. The task is difficult and we will use the recommendations in the report to support further improvement over the next 12 months."
A Prison Service spokesman said: "Women and men are normally transported separately, but for operational reasons it is sometimes necessary for them to travel together.
"In exceptional circumstances when they are transported in vehicles without partitions, cases are individually considered and any potential risks are carefully managed."