Ministry of Defence staff were warned before the Afghan data leak not to share information containing hidden tabs, according to documents released by the UK's data regulator.
Last month it emerged that the details of almost 19,000 people who had applied to move to the UK were leaked when an official emailed a spreadsheet that contained a hidden tab with the information.
Documents released by the Information Commissioner's Office (ICO) also show that staff there raised concerns about why the body had not issued a fine to the MoD.
The MoD said they had worked to improve data security, but an ICO spokesperson said the government had not yet done enough to learn the lessons.
According to an ICO memo, guidance in place at the time of the leak showed that the "MoD was aware of the risks of sharing data and explicitly referenced the need to remove hidden data from datasets".
