Christopher Clark, 46, pounced on the woman, placed a plastic bag over her head and indecently assaulted her yards from a bail hostel in Bath where he was being held on probation.
As Clark began his life sentence, probation officers said they had been "waiting for something to happen" as soon as he was freed after serving nine years of the14-year term.
But they were helpless to prevent his release because his original sentence for rape was handed down before the 1991 Criminal Justice Act made it possible to keep offenders inside if they are still considered a risk to society.
Members of Clark's family revealed that they had pleaded with the authorities to prevent his release. And Don Foster, the Liberal Democrat MP for Bath, said the law should be tightened to prevent other dangerous offenders sentenced before 1991 being released.
Gary Redfern, Avon Probation Service's assistant chief officer, said: "There was nothing we could do and we were simply waiting for something to happen - we considered him to be dangerous. His offending had not been addressed and he had not changed. We knew Christopher Clark as a risk."
Mr Redfern defended the service which he said had been unfairly criticised in the case. Clark was considered too dangerous to be released on parole, but, under the rules operating at the time, was entitled to remission. He had been ordered to take the drug Goserelin to destroy his sex drive.
Clark, who was nicknamed the "early bird rapist" following a string of knife-point sex attacks on women in the 1980s, had denied attacking the 23-year-old teacher.Reuse content