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They can now be accessed by anybody, anywhere in the world - although there will be a small fee to do so. A further two million records will be going online by the end of the year. Among the documents are court papers, verdicts, sentences and mugshots. Many of the photographs show suspects holding their hands to their chests, in case they have an identifying feature like a missing finger. Family historian Amy Sell told Sky News: "You may get their age or a description of what they looked like. "You may even get a photograph as well, which is amazing. Coming face to face with your ancestor is quite rare. "You will also know what they were convicted of, what the crime was, and also what the punishment was." Two-and-a-half-million documents are being painstakingly scanned by staff at The National Archives in Kew. Those interested in tracing their family history can search the files by entering names and keywords. But the documents also provide a fascinating insight into the criminal justice system at the time, and wider social history. Records specialist Paul Carter says we do not have much detail about ordinary people's lives in the 18th and 19th centuries. "What people should be looking for - as well as the murderer or the thief - is the rural poacher, or the early trade unionist at a time when trade unions were illegal," he said. "You'll get Chartists in here and people who were trying to reform the way society worked. So you are going to get an awful lot of very different people who come through the system, for different reasons." You can also look up the criminal records for more well-known names like the political activist Emmeline Pankhurst and author Oscar Wilde.

Family history website Find My Past has launched a new service that allows you to dig up your dodgier ancestors. The initial set of Find My Past covers 1817 to 1931, but it will eventually be possible to search back to 1770. I know virtually nothing about my relatives, but had a grisly feeling some might be highly disreputable. Was a dip into my family's past really wise?

Apparently, my inhibitions are not widely shared. "People are very excited by criminal ancestors," says Debra Chatfield, marketing manager for the site. "When we did a survey asking what sort of relatives they hoped a search would throw up, black sheep came out No 1, way ahead of royalty or aristocracy. People have a romantic notion of vagabonds, especially in the Victorian period." It seems that what you don't want to find is that you hail from a long line of accountants or quantity surveyors.

It soon became apparent that starting a family search by looking for the black sheep is putting the (probably stolen) cart before the (almost certainly abused) horse. You really need to know who your ancestors are before you try to find out whether they had a criminal record. The only Victorian relative whose name I know is my grandfather, Albert Moss, born in the Welsh town of Newport in 1892. I'm happy to say he did not fall foul of the law.

But I'm a little worried about Thomas Moss, a seaman born in 1849, who on 22 November 1895 in Newport "did unlawfully obtain money, food and lodgings from Catherine Keefe by false pretences with intent to cheat and defraud". This may just be a coincidence – the indictment viewable on the website shows he had a string of other convictions for frauds committed in Cardiff and Liverpool – or he may be my own Victorian vagabond. Finding out will require a lot more digging. Thomas Moss is a common name – there were a dozen born in the UK in 1849.

Unlocking the past comes at a price (financial rather than psychological). A full subscription allowing you access to all the documents on findmypast.co.uk can cost up to £110 a year. But it has whetted my appetite. I would like to discover who and what my ancestors were. Even the fraudsters. Maybe even the quantity surveyors.

Chatfield had her own epiphany when researching the criminal records. "I found a great-great-uncle in the Victorian prison hulks," she says. "He'd stolen 18lb of cheese. I felt sorry for him. He was only 18 and ended up on a prison hulk for seven years. I'm partial to cheese myself, so I can see where he was coming from."