The Ministry of Defence has launched an investigation into how the identities of 250 Afghanistan-based interpreters who worked with UK forces were compromised in a group email.
Their addresses and other identifying information are understood to have been mistakenly copied into an MoD communication sent by the team in charge of the UK's Afghan Relocations and Assistance Policy.
The ministry declined to give specific details of the breach but said people affected were being offered advice on how to manage potential risks posed by the error.
Shadow defence secretary John Healey said the MoD had “needlessly put lives at risk” among a group the UK was committed to protecting following the return to power of the Taliban last month.
“The priority now is to urgently step up efforts to get these Afghans safely to the UK,” he said.
“This is the second major data breach from the MoD this year, after sensitive documents were discovered at a bus stop in Kent in June. Clearly, the defence secretary needs to get his house in order.”
An MoD spokesperson said: “An investigation has been launched into a data breach of information from the Afghan Relocations Assistance Policy team.
“We apologise to everyone impacted by this breach and are working hard to ensure it does not happen again.
“The Ministry of Defence takes its information and data handling responsibilities very seriously.”
The BBC, which reported the data breach first, said the MoD appeared to have realised an error had been made soon after the original email was sent. It said a follow-up message had been dispatched 30 minutes later warning recipients that their details may have been compromised and advising them to delete the earlier message.
Last month the Foreign, Commonwealth and Development Office drew criticism – including from defence secretary Ben Wallace – after it emerged that officials leaving the British Embassy in Kabul had failed to destroy sensitive documents that identified Afghan employees and would-be staff.
The documents, said to provide names, addresses and phone numbers for seven staff, were found in the garden of the embassy by The Times.
Wallace said at the time said he expected prime minister Boris Johnson to be “asking some questions” about the lapse.
“We’ll find out and get to the bottom of it. The evidence looks pretty clear. Clearly it’s not good enough, simple as that… I think we need to understand, quite rightly, how that happened,” he told Sky News.