Former BBC journalist Christopher Graham took up his post in June
Laws should be tightened to give judges the option to jail people found guilty of serious abuses of personal data, the UK's Information Commissioner has said.
Christopher Graham said penalties under the Data Protection Act were "pathetic" and urged tougher laws by next April.
His comments came after a judge said it was "ridiculous" he could only fine a former British National Party activist for leaking its membership list online.
Mr Graham is preparing a dossier to present his case to ministers.
He told BBC Radio 4's Today programme: "I think for the most blatant and reckless examples of people playing fast and loose with private information, a custodial sentence is certainly appropriate.
"The DPA is supposed to safeguard confidential personal information, but the penalties in the Act are simply inappropriate for the activities going on today.
Private investigators can offset fines against expenses, Mr Graham says
"There is a very lively trade in confidential personal data and if the only thing you are going to get as a private investigator is a pathetic fine in the magistrates' court and even the judge is embarrassed to impose it, it is simply not enough."
He said the worst cases included trying to find out about partners' assets for divorce proceedings, jury intimidation and jury nobbling.
Others find out where people live by getting hold of ex-directory numbers and friends and family numbers, checking the police national computer or car licensing database, he said.
Mr Graham, who took up his post in June this year, also told the BBC private investigators who were given small fines could set the penalty off against expenses.
Former BNP member Matthew Single, 37, of Brinsley, Notts, was fined by Nottingham magistrates after admitting publishing contact details of 10,000 members.
The leak led to complaints of harassment and threats.
In court, judge John Stobart said: "It came as a surprise to me, as it will to many members of the party, that to do something as foolish and as criminally dangerous as you did will only incur a financial penalty."