Personal details and important information have been discarded in a city centre waste bin.
The papers, revealing names, addresses and national insurance numbers, were found at the back of a post office in London Road, Southampton.
The company which made the discovery, scamsdirect.com, said people had been left vulnerable to identity fraud.
The Post Office has apologised and said an investigation was under way to ensure the situation is not repeated.
Paul Witts, from scamdirect.com, said: "This sort of information is very sensitive and given all the advice that people like the Post Office and the banks give you to actually get rid of this information and shred it yourself, they're not following their own advice - it's scandalous!"
Scamsdirect.com seeks to expose malpractice by individuals or organisations.
The company's work can involve sifting through skips and rubbish bins looking for discarded personal documents.
A quarter of adults are affected by ID theft in the UK and the Home Office estimates that identity fraudsters steal £1.7bn each year.
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