More than 77 million historical records, including details of Victorian workhouses, are being published online.
The London Historical Records traces 165 million people through various records dating back to the 1500s.
The records shows author JK Rowling's family, Oliver Cromwell's marriage and Britney Spears's great-grandfather, whose marriage is listed in Tottenham.
Currently 250,000 records are visible at ancestry.co.uk with the rest to be uploaded by the end of 2010.
The final collection will include parish and workhouse records, electoral rolls, wills, land tax records and school reports.
Among the celebrities found in the records are David Beckham's family going back to his great-great-great grandparents, Spears's great-grandfather George Portell in 1923 and Rowling's great-great-great-grandfather William Richard Rowling in 1872.
We estimate that half of Brits will be able to find an ancestor in this collection
Josh Hanna, Ancestry
Apart from Londoners, about 135 million people from the US, Canada, former colonies of the British Empire and Australia may be able to trace their ancestors.
Details about Victorian workhouses from Board of Guardians records covers 12 areas and lists anyone who was born, baptised or died in a London workhouse in the 19th and early 20th century.
The collection was digitised by City Hall along with London Metropolitan Archives and Guildhall Library Manuscripts.
Josh Hanna, senior vice president of Ancestry, said much of the collection pre-dates civil registration and censuses.
"We estimate that half of Brits will be able to find an ancestor in this collection, which documents the history of a great city and its people.
"No city in modern history other than London can claim to have been the capital of such a far reaching empire, which really is why this collection is of such significance not only to Brits, but also to many others around the world with ancestral ties back to England."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.