About 500,000 amateur family historians flocked to the Ancestry.co.uk website after it unveiled an online version of the UK's first comprehensive census from 1841.

Visitor numbers to the genealogy website almost quadrupled from normal levels as people logged on last Monday to see what their great-great-great-great grandparents had been doing on a particular night 160 years ago.

Ancestry.co.uk, part of MyFamily.com, the privately held US internet group, is one of the fastest-growing number of websites that have emerged to cater for the surge in interest in researching family history over the internet.

According to ComScore, the market research company that tracks internet usage, visits to family history websites have almost doubled over the past year in the UK, with more than 4m visits in January against 2m in January 2005.

This is at a time when the total internet audience is estimated to be growing at about 7 per cent each year.

Ancestry.co.uk and related websites that are part of the same group, such as Ancestry.com and Rootsweb, have seen 142 per cent growth year-on-year, now reaching an audience of 2.4m.

Genes Reunited, part of the Friends Reunited group of websites and still the UK's largest single genealogy site, saw the number of visitors each month increase more than 30 per cent to 1.6m between October and January.

Josh Hanna, managing director of Ancestry Europe, says there has always been interest in researching family trees but the prospect of going to dozens of parish records offices for research has put off many would-be family historians.

Putting these records online, however, has given people a quick and easy way to gain access to them.

The popularity of online historical records was first tested in 2002 when the Public Record Office put the 1901 census online. There was unprecedented interest in the records and the site crashed after just three days of operation under the weight of 32m hits a day.

More recently, programmes such as the BBC's Who Do You Think You Are, which in 2004 explored the family histories of celebrities including Jeremy Clarkson and David Baddiel, have played a large part in driving interest in genealogy.

Martine Parnell, head of Genes Reunited, says interest in the site accelerated sharply in late autumn 2004 when the show was aired. Normally, 120,000 people a day log on to Genes Reunited, but this climbed to 160,000 in the middle of the six-week series.

Genealogy sites are a lucrative business. Ancestry has about 800,000 paying members worldwide, including 150,000 in the UK, most of whom pay about £70 for annual membership.

This gives the company more than $140m in annual turnover, a figure that has tripled in the past year. The business has been profitable for the past three years.

Genes Reunited allows an individual to register search archives for free but charges a £9.95 annual subscription to enable users to send messages to other members. It also charges users for viewing actual records, such as birth, marriage and death certificates and census entries, with 10 to 50 records costing about £5.

Genes Reunited does not publish separate revenue figures but Ms Parnell says sales increased 70 per cent in 2005 and are expected to double this year.

The charges levied for looking at records online are a common source of grumbling among online genealogists, but ultimately most of them are paying up, a fact that has caught the attention of the big media companies.

The Friends Reunited group, for example, was bought by ITV last December for £120m.

Ancestry, which is backed by venture capital groups including CMGI's @Ventures and Spectrum Equity Investors, says it has also received a number of approaches from big media companies but has turned these down.

"We are looking at all options, including a float, to staying private to a sale," Mr Hanna says. He sees great potential for genealogy sites to keep expanding at a rapid rate.

Ancestry, for example, is trying to appeal to a younger audience. The typical user tends to be in their 40s and 50s, but Ancestry is looking for ways to address different age groups - such as young mothers who may be inspired to look into family history after the birth of their children.