What documents must employers keep to prove right to work?

For a UK employer to have a statutory excuse against an illegal working penalty, they must have taken a copy of an acceptable right-to-work document before employment started, checked it in the presence of the employee (or via the Home Office online checking service for those with an e-Visa), and retained the copy securely. Acceptable documents are listed in the Home Office's Lists A and B. List A documents (British passport, EEA passport for those with settled status) provide a permanent excuse; List B documents (limited leave to remain) require a repeat check.

The copy must be clear enough to identify the person and show the document's validity dates. Digital copies are acceptable (scanned or photographed). Crucially, employers don't need to retain the physical document -- just a clear copy. The copy must be kept for the duration of employment and 2 years after employment ends, so auditors can verify compliance retrospectively.