Robert Bailey, 73, of Belmore Close, Fratton, was jailed for 23 years for abusing five young girls.
Now a lawyer acting for the victims said it is a ‘distinct possibility’ that more victims may exist.
At Portsmouth Crown Court one victim, who was subjected to a ‘campaign’ of abuse branded Bailey a ‘monster’. The pensioner had raped her repeatedly after ‘softening her up’.
Victoria Neale, of Hudgell Solicitors, who is representing victims, said Bailey’s actions were ‘premeditated and opportunistic’.
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She said: ‘There are many factors in this case which appear to make it a distinct possibility that there will have been more victims of Bailey,’ said Mrs Neale.
‘This is something my clients have specifically raised with me, particularly following his conviction of indecency with one child whom he picked up in his lorry.
‘There seemed to be both premeditated campaigns of abuse, but also the opportunistic nature to his offending in picking up a girl when driving.
‘It is also clear that he targeted very young girls, girls who were so young that they may not have realised they were victims at the time.
‘Our clients have each kept this abuse to themselves for many years, and had actually been threatened by him should they speak out.
‘Now that he has finally been jailed for these despicable acts, we hope that should there be any others who have suffered at his hands, they will now feel able to come forward, as did those victims so bravely who brought him to justice at trial.’
Overweight naturist Bailey, who is more than 6ft, abused one girl as young as two, the court was told.
Bailey carried out the string of attacks in the 1970s in Hampshire.
Sentencing, judge David Melville QC said: ‘These are probably one of the most serious series of events that one could possibly imagine.
‘Five young girls have been suborned by you in various ways, their lives have been wrecked — and you are responsible for that.’
Bailey was convicted at trial five indecent assaults, two rapes and three counts of indecency with a child.
He had denied all of the charges at trial.