Defence Secretary Liam Fox has apologised to 38 soldiers who heard that they were being made redundant by e-mail, including one who is currently serving on the front line in Afghanistan.
Dr Fox said the situation was "completely unacceptable", and also told MPs he regretted the way trainee RAF pilots had discovered they were to lose their jobs.
Having been summoned to the despatch box to answer an urgent question in the Commons from shadow defence secretary Jim Murphy on 15 February 2011, Dr Fox said: "As a result of the Strategic Defence and Security Review and the Comprehensive Spending Review, it has sadly been necessary to plan for redundancies in both the civil service and armed forces.
"At all times this should be done with sensitivity to individuals concerned and with an understanding of the impact this will have on them and their families."
Referring to the e-mails, he said: "This is a completely unacceptable way to treat anyone, not least our armed forces. The correct procedure was not followed; I regret this and want to reiterate the unreserved apology already made by the Army and on behalf of the Ministry of Defence.
"Arrangements have already been put in place to make sure this does not happen again and the Army are already investigating the particular circumstances."
It was reported on Monday in the Daily Telegraph that a quarter of the RAF's trainee pilots - up to 100 - are to be sacked.
Dr Fox told MPs: "The fact that they found out through the publication of very inaccurate detail in a national newspaper will I'm sure be deprecated by both sides of the House and can only cause the individuals concerned undue distress."
Mr Murphy said: "We all know that you cannot stop every redundancy in the armed forces but this is no way to treat soldiers who have served in Northern Ireland, Balkans, Iraq and Afghanistan.
"Sacking anyone by email is always wrong, sacking our armed forces in this way is utterly unforgivable."