The National Association of Probation Officers (Napo), the officers' union, is consulting lawyers over whether some of the searches amount to assault.
"Rub-down" searches, which have included examinations of ears, mouths, hair, inside legs, shoes and breast and waistband areas, were introduced last autumn.
By this month more than 60 officers, the majority women, had formally complained to the prison authorities.
The instructions at Belmarsh high category prison, in south-east London, says that the searches of probation staff are "designed to give a thorough examination to satisfy the searcher that no item is concealed in their clothing." There is no evidence, however, of probation staff being involved in illegal activity.
A parliamentary answer in 1989 disclosed that 10 prison officers and other staff members had been interviewed in connection with drug trafficking in prisons during the previous decade. While the number of arrests for drug smuggling by visitors has rocketed to 1,289 in 1995, the Prison Service has stopped differentiating between domestic and professional visits when compiling its statistics.
Harry Fletcher, Napo's assistant general secretary, said: "We suspect that the real reason for the searches is connected with the Prison Service's attitude to probation officers. At one jail a female officer was told that probation was seen as the friend of the prisoner and therefore likely to take in contraband."
A Prison Service spokesman insisted that random pat-down searches of all visitors had been introduced in response to the Woodcock report into the jail escape from Whitemoor, Cambridgeshire, in September 1994. "Probation officers spend a lot more time going in and out of prisons," the spokesman said. "Our outstanding consideration is security."
Three recent cases in particular have prompted the union to seek advice on whether officers have been subjected to assault or grossly unreasonable treatment. In one incident at Belmarsh, a female prison officer put her fingers inside a female probation officer's bra and leggings, observed by a group of male solicitors and a female solicitor who were not searched.
In another search at the same prison, a male prison officer approached a female probation officer with a hand-held electronic metal-detecting wand, but the woman was instead subjected without warning to a search under her clothing by a female prison officer. Napo views both incidents as assaults.
In the third case, at Doncaster prison, the searching officer insisted on touching a female probation officer's ileostomy bag, in front of onlookers, asking what she had in her pockets.
Napo believes that police and immigration officers are far more likely to be "waved through" by jail security. Mr Fletcher said dozens of officers had walked out of prisons in protest and judges were now expressing concern because pre-sentence court reports were not being completed.
In other cases officers have reported being sniffed by drugs dogs, while prison authorities have even considered asking women not to wear underwired bras because they set off alarms.Reuse content