The Army has apologised after 38 soldiers learned they were losing their jobs by e-mail.
The men - all long-serving warrant officers and including one working in Afghanistan - were told they were victims of the defence cuts.
The Army has said sorry for the distress, adding that commanding officers have since lent them support.
Defence Secretary Liam Fox said what happened was "completely unacceptable" and the Army was investigating it.
Speaking in the House of Commons, Dr Fox also said he regretted the way trainee RAF pilots had discovered they were to lose their jobs.
On Monday, the Daily Telegraph reported that a quarter of the RAF's trainee pilots - up to 100 - are to be sacked.
Meanwhile, the Sun said the soldiers each received a message giving them one year's notice and advising them to "start planning your resettlement".
It said the warrant officers included a Royal Tank Regiment veteran who was working on the front line in Afghanistan when he received the redundancy notice.
All of the soldiers have completed several decades' service and continued in the Army on a rolling contract called the long service list.
The e-mails should have gone to the men's commanding officers, who would normally deliver such sensitive news face-to-face. Instead, the Army Personnel Centre sent them directly to the soldiers on 11 January.
Maj Gen Richard Barrons, the Army's assistant chief of general staff, said: "It's a terrible mistake and I can only apologise on behalf of the Army to the 38 soldiers concerned, their families and the rest of the Army who would have been concerned about this story.
"It should not have happened and we plan for it not to happen again."
He said the Army Personnel Centre was examining its procedures, checks and balances, and staff training.
David Cameron's spokesman said the matter's handling was "unacceptable".
Dr Fox, who faced cries of "Resign!" from Labour backbenchers as he stood to answer an urgent question from shadow defence secretary Jim Murphy, said the Strategic Defence Review and Comprehensive Spending Review had sadly made redundancies necessary, but they should be handled with "sensitivity to individuals concerned".
Referring to the emails, 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."
On the trainee pilots' redundancies, he said: "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."
But he added: "It was always going to be the case that with fewer airframes, we would need fewer pilots."
Mr Murphy said it was "a shame" Dr Fox had had to be "summoned" to the Commons to report on the redundancies when he could have made a statement voluntarily.
"We all know that you can't stop every redundancy in the armed forces but this is no way to treat soldiers who have served in Northern Ireland, the Balkans, Iraq and Afghanistan.
"Sacking anyone by email is always wrong; sacking our armed forces in this way is utterly unforgivable," he said.
Mr Murphy said a "pattern" was developing after trainee pilots were "sacked by media leak".
"What's worse about this sordid affair is that the government's response has been to blame everyone else.
"In the morning, it was the Army's fault; by lunchtime, it was the civil servants' fault. But it wasn't the Army that decided to cut the deficit this far and this fast," he said.
'Protection step change'
Up to 100 RAF trainee pilots are expected to be told on Tuesday they have to leave.
Those said to be facing the axe because of defence cuts are up to 20 fast jet pilots, 30 helicopter pilots and 50 transport aircraft pilots.
Plans are to reduce RAF personnel by about 5,000 to a total of about 33,000 by 2015.
The MoD has said any reductions would not affect operations in Afghanistan and "priority areas of capability" would not be compromised.
Last October, the government's strategic defence and security review (SDSR) outlined cuts of £4.7bn over four years.
Unveiling the review, Prime Minister David Cameron said defence spending would fall by 8% in that period.
He said it was not a "cost-saving exercise" but a "step change in the way we protect this country's security interests".
Some 42,000 defence jobs will be cut by 2015 - including 25,000 civilian staff at the MoD, 7,000 in the Army and 5,000 in the Royal Navy as well as the RAF.