Between April 2016 and March this year, 8,194 soldiers joined the British Army.
However, 9,775 left during the same period, with family life and “opportunities outside the forces” among the reasons given.
This leaves the MoD 5.1% below its Army target, compared with 4.8% in the RAF and 2% in the Navy and Royal Marines.
A document released by the department on Thursday told: “There is no single is why personnels leave on voluntary outflow.
“But the personnel who accomplished the armed forces continuous attitude survey indicated the rationale for leaving the armed forces included the impact of service life on family and personal life, and opportunities outside the armed forces.”
A spokesman from the Army said the force had enough people to “meet all our current commitments at home and abroad”, and told recruitment amounts were on the rise.