Some UK combat regiments are operating at almost 40% less than their required strength due to declining recruitment numbers, according to data.
Figures from the MOD obtained by The Guardian under Freedom of Information (FOI) laws revealed numbers in Army infantry units have been consistently falling over the past five years.
Information publicly published quarterly by the ministry noted a 7.6% drop in Army personnel on 1 January.
But The Guardian's analysis of the FOI figures shows nine of 16 infantry regiments were 20% below their required personnel level, four were 25% below and the prestigious Scots Guards unit was 37% below its target strength.
The infantry as a whole was operating at 17% less than its required level, the paper added.
Defence Secretary Ben Wallace has been urged to address the issue.
Shadow defence secretary Nia Griffith was quoted as saying the "shocking trend" had to be halted.
"At a time when our country faces an increasing number of threats, it is simply unacceptable for numbers to be falling year after year," said Ms Griffith.
The MOD said the Army "continues to meet all of its operational commitments to keep Britain safe".
An Army spokesperson said "applications to join the Army are at a five year high, with around 77,000 applications to join as a regular soldier alone" in the 2018/19 financial year.
The spokesperson also said the Army have also "increased the enlistment to conversion rate from one in 10 to one in eight."
"We are fully committed to improving our recruitment process and we are working with Capita to address remaining challenges."
A Capita spokesperson said: "The partnership is in the middle of a well-publicised re-set, which started last year but is already seeing excellent results.
"This includes the busiest quarter since the partnership started seven years ago, with 1,000 more enlistments in January to March than the same three months last year; a successful trial to halve the amount of time to get recruits from application to basic training; and increasingly high proportions of applicants getting an offer of basic training, which is substantially better than pre-2012 [when Capita started the contract]."