The Ministry of Defence "cannot take future recruitment for granted", a minister said, as it emerged the Army missed its target last year by more than 2,500 troops.
New figures released to Parliament show 6,910 new regular troops joined the Army over the past 12 months, against a target of 9,580 - a shortfall of 28%.
The biggest deficit was in the infantry, where 2,380 troops were recruited against a target of 3,480.
The figures were released after a written parliamentary question by Labour MP and former defence minister Kevan Jones, who suggested ministers could be using this lack of manpower to balance the defence budget.
In a separate question from Labour peer Lord Touhig, defence minister Earl Howe said the Ministry of Defence (MoD) "cannot take future recruitment for granted" in the wake of a national skills shortage and a strong, growing economy.
The Army's £500 million recruitment contract with Capita has been heavily criticised in the past, amid IT problems and repeated failures to hit targets.
Mr Jones said: "These figures show just how serious the Army's manning shortfall is and ministers need to clarify whether or not this is a deliberate attempt to balance the defence budget.
"No action on this issue will just lead to further overstretch for our armed forces and in turn even worse recruitment and retention."
The figures relate to untrained recruits starting their phase one training.
In his answer to Mr Jones, defence minister Mike Penning said between December 2015 and December 2016, 633 officer recruits joined the Army's untrained strength, against a target of 745.
Mr Penning added: "Targets are internally set by the Army recruitment and training division and are subject to constant change and review."
The Army was cut from 102,000 to 82,000 regulars after the 2010 defence review, while the number of reservists was promised to increase from 19,000 to 30,000.
The Army hopes to achieve the 82,000 target before 2020.
Responding to Lord Touhig's question about shortfalls in recruitment to the armed forces, Earl Howe said there were currently a number of campaigns running to boost enrolment.
He added: "However, a national skills shortage and a strong growing economy mean that we cannot take future recruitment for granted.
"In response we have in place a number of short and long-term plans to ensure that the offer of military service in the armed forces continues to be competitive, so that we can recruit and retain in sufficient quantity to meet the strength targets set out in the strategic defence and security review 2015."
Official statistics released by the MoD last week show that as of December the armed forces are 4.3% down on the planned number of personnel needed, slightly worse than the same time last year.
Excluding any redundancies, the figures show 15,140 regular troops left the armed forces over the past 12 months, with 13,450 new troops coming in.
Last month Defence Secretary Sir Michael Fallon defended the recruitment partnership with Capita, after the Reserve Forces' and Cadets' Associations (RFCA) called for an urgent contract review.
Responding to the RFCA's external scrutiny report, Sir Michael said: "I recognise and understand the team's concerns about the Army's recruit partnering project.
"This is a critical programme, which has experienced considerable challenges and which has yet to deliver to full capability.
"It is a programme that we keep under review at the highest levels.
"However, considerable progress has been made to improve performance, both by the contractor, Capita, and by the Army and we will continue to review delivery and performance of this contract."