Ancestry users can now trace family members on the eve of the Second World War after the website digitised the England and Wales 1939 Register.
It was compiled in September 1939 after war was declared in order to have an accurate record of Britain's civilian population to enable issuing of ration books and ID cards, the direction of labour and conscription into the armed forces.
The Register contains the names of over 45 million people, listed by address, along with their date of birth, their job and in some cases details of their voluntary war work.
Images of the original records, held in The National Archives, are available to search through a new index created by Ancestry.
Unlike Findmypast's version, the index includes information from Column 5 of the original form, which marks individuals who were resident in hospitals, asylums or prisons with the letters O, V, S, P and I, standing for Officer, Visitor, Servant, Patient or Inmate.
Russell James of Ancestry described the Register as "the only record which can tell us invaluable information about the lives of our close family - such as grandparents or parents - as Britain entered World War II".
The 1921 census cannot be released until 2021, while the 1931 records were destroyed during the war and no 1941 census was taken owing to the ongoing conflict.
As such, the 1939 Register is the only complete online record of the population of Britain after 1911.
Some images have been redacted to protect the privacy of those who may still be alive.
Every year, Ancestry will publish the records of individuals who were born more than 100 years ago, or whose record of death has been reported to The National Archives.
Researchers can request extracts from the register for Scotland and Northern Ireland from the National Records of Scotland and the Public Record Office of Northern Ireland, while records for the Isle of Man and the Channel Islands have not survived.
The 1939 Register also reveals how much women's lives changed during the Second World War.
The most popular occupations listed for women are 'Unpaid domestic duties' and 'Paid domestic duties', with other common professions including 'Housekeeper', 'Shop assistant' and 'Dressmaker'.
However, women would soon be recruited into traditionally male spheres in order to help with the war effort.
By 1943, almost 90% of single women and 80% of married women were working in factories, on the land or in the armed forces.
The records also list the names of famous people of the period.
Winston Churchill is recorded as living in Westminster along with his wife Clementine, with his profession given as '1st Lord of the Admiralty'.
However, the record has subsequently been amended to read 'Prime Minister' after Churchill replaced Neville Chamberlain on 10 May 1940.
The 1939 Register continued to be updated until 1991, with new details added such as women's married names.
Other notable people found in the 1939 Register include the authors George Orwell (under his real name of Eric Arthur Blair), Virginia Woolf and A. A. Milne; former suffragette and political campaigner Sylvia Pankhurst; and future Bletchley Park codebreakers Alan Turing and Alfred Dillwyn Knox.
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Rosemary Collins is the staff writer of Who Do You Think You Are? Magazine