A service showing the 1861 census for England and Wales is being made available online for the first time.
Interest in family history is booming
The company behind the initiative hopes it will interest the growing number of people trying to trace family history.
The firm, 1837online.com, which will charge users on a pay-per-view basis, is putting the whole census online, starting with south east England.
Until now, the data has only been available at the Family Records Centre in London or at certain libraries.
1837online.com managing director Colin Miller said the census was "incredibly useful if you're trying to do your family history" as it enabled users to find out not only where their ancestors lived, but who they lived with.
The census also shows people's ages, occupations and disabilities.
It will cost a minimum of £5 to buy a block of 50 units of credit for the website. One unit counts for one look at a births, marriages and deaths page. Looking at one household's census costs three units - or 30p.
Mr Miller said the growing interest in genealogy stemmed from people wanting to get a sense of who they were and where they came from.
He told BBC News: "It's normally sparked off by a family event such as a birth or, unfortunately, a death.... It's almost like doing a bit of private detective work on yourself."
The 1861 census is the oldest census to go online so far and includes data about famous people such as Florence Nightingale.
Interest is expected to be keen - when the 1901 Census was put online on the National Archives website in 2002, the site crashed under the pressure of 30 million hits per day.
The 1861 census is available for free on microfilm in the National Archives and the Office of National Statistics Family Records Centre in London and in local record offices and libraries.
The National Archives plans to put all the historic census records from 1841-91 on its website within the next two years.
Census records date back to 1801 and are carried out every 10 years.
They were introduced to provide information on Britain's expanding population.