Many prostitutes face being jailed for up to 72 hours if they fail to attend counselling sessions under proposed new laws, a probation service union claims.
New legislation may see prostitutes detained for three days
Napo said the Criminal Justice and Immigration Bill reintroduces custody by the "back door" and has urged MPs to delete the measures from the Bill.
It says prisons, community homes and police stations could be "overwhelmed" by women in default of their orders.
Ministers say it is only one of a range of measures to tackle prostitution.
Under the proposed law, women convicted of loitering and soliciting could be ordered to attend three one-hour counselling sessions.
And they may get three days' detention for not attending.
A Napo briefing note on the bill said: "Thousands of prostitutes will be criminalised and face three days needlessly in jail at the time when the system is in meltdown."
Harry Fletcher, Napo assistant general secretary, said: "The causes of prostitution are complex.
"The vast majority of women have no obvious alternative source of income or employment and the majority have severe problems with drugs and alcohol.
"Three compulsory counselling sessions are hardly going to help. The threat of a short period of incarceration for failure to turn up turns the clock back 25 years.
"If the probation service or other organisations are to be involved they must have the resources to offer the women real alternatives."
A government spokesman said: "Counselling sessions for women convicted of loitering or soliciting would not necessarily be undertaken by the Probation Service.
"That would be designated by the court and could also be done by a local support network for prostitutes or a drug treatment service.
"Three days is the maximum that any offender would spend on remand - the vast majority of cases would be dealt with in one day.
"This is one of a range of measures to tackle this issue by getting prostitutes out of the criminal justice system and into support and treatment."
MPs will debate the Bill in the Commons on 9 January.