The handling of data in the Ministry of Defence is “not good enough”, Ben Walllace admitted after a second blunder was revealed in the team responsible for relocating Afghan allies to the UK.
The Defence Secretary told the PA news agency he was “pretty angry” about the situation.
But he said an investigation had been launched and steps had been taken to “make sure this doesn’t happen again”.
The BBC reported that dozens of people who may be eligible to come to the UK under the Afghan Relocation and Assistance Policy (Arap) were mistakenly copied into an email earlier this month.
However, their email addresses were visible to all recipients, rather than being blind-copied to protect their identity.
Under the Arap scheme any Afghans who assisted UK efforts in Afghanistan, such as interpreters, can apply to come to the UK due to their risk of persecution at the hands of the Taliban.
There are fears that they may be put further at risk by the breach if the information falls into the wrong hands.
The saga is the second of its kind after Mr Wallace told MPs on Tuesday a staff member had been suspended for the first breach.
The BBC reported the details of 55 people were included in the email referred to in the latest investigation.
Mr Wallace told PA during a visit to Rosyth the second breach actually predated the first one that was revealed.
“That’s an older breach from the one I originally came to Parliament to announce,” he said.
“Subsequently I’ve kicked off an investigation, I was unaware of this earlier, smaller breach, it’s not good enough, I’ve said that not only was I pretty angry at the time but also we’ve got to have an investigation and fix it.
“At the same time I’ve committed even more resource and people to make sure this doesn’t happen again and look after those people in Afghanistan we’ve brought back over 8000 people on the Arap scheme, that’s a significant number of people including their families dispersed around the United Kingdom but there are 260 people left behind that we have to do our very best to get out.
“Those are the people, some of them, are concerned by the leak quite rightly, we will do everything we can to make sure they are protected and then work with neighbouring states to get them out.”
On Tuesday Mr Wallace issued a House of Commons apology and told MPs he “immediately directed investigations” take place after the previous breach involving more than 250 people.
Mr Wallace later confirmed Admiral Sir Ben Key, the commander of joint operations who led the planning and evacuation from Kabul is leading the investigation and one person had been suspended.