Nearly 2,000 armed services personnel are learning that their jobs are to disappear today.
At least half of the 920 Army jobs and 930 RAF roles are expected to face compulsory redundancies in the first round of defence cuts.
RAF pilots prepare for a mission over Libya.
The Gurkhas face the heaviest cuts, with 140 soldiers facing redundancy. No service personnel currently receiving the operational allowance will be forced to leave, however.
"Just as many RAF personnel will be thinking about returning home having performed heroics in Libya, ministers are drawing up their P45s," shadow defence secretary Jim Murphy said.
"People will be shocked and will wonder whether the government have got their priorities straight."
The defence cuts follow an eight per cent overall reduction in the MoD's budget in the comprehensive spending review.
Broader financial problems have forced defence secretary Liam Fox to seek a more fundamental rationalising of his department, leading to plans for over 22,000 jobs to go in total by 2015.
"The tough measures will bring the budget largely into balance for the first time in a generation," Mr Fox said.
"Extra money allocated for the equipment budget from 2015 will allow capability to grow in the second half of the decade."
The slow-burning nature of the cuts makes them politically sensitive, however. Around 1,600 redundancies in the Royal Navy are expected to be announced next month.
"This underlines the scale of the government's cuts in manpower. The frontline cannot be protected from cuts this deep," Mr Murphy added.
"Savings must be made, but in a world of uncertainty to many this will seem a worrying loss of important capability."