Service and pension records for more than two million soldiers who fought in the British army in World War I are being put online for the first time.
The documents provide a broad range of detail, from name and next of kin to wounds suffered and conduct record.
The release by the Ancestry website, working in partnership with the National Archives, is taking place in stages over the next two years.
The images are available to view on a subscription or pay-per-view basis.
All the records are already viewable on 28,000 rolls of microfilm at the National Archives in west London, but it is hoped the digitisation process will make them available to a much wider audience.
The National Archives described the online release as "tremendously significant" and said it would lead to a better understanding for military historians as well as help those researching their family trees.
Although five million soldiers from England, Scotland, Wales and Ireland fought in World War I, about 60% of the service records were destroyed in a German bombing raid in 1940.
The surviving records, many badly damaged and known as the "burnt documents", were conserved by the National Archives and filmed.
The pension record details for about 100,000 soldiers are the first to go online.
These relate to some of those men discharged on account of sickness or injuries sustained and include the medical records relating to the disability for which a pension was granted.
The service records, which will follow, describe the careers of soldiers who completed their service, were killed in action, executed or died of their wounds or disease, and provide full details of their service, and, where recorded, death.
The number of documents relating to a soldier varies according to their circumstances, but in some cases there are scores, including items of correspondence.