The Armed Forces are facing a shortfall of almost 6,000 soldiers, sailors and airmen, according to official Ministry of Defence figures.
The latest manning quarterly figures show the total number of full-time, trained military personnel was 173,370 - 5,790 short of the total requirement of 179,160.
The Army had a 3,500 shortfall, the Royal Navy was 1,220 under strength, while the RAF was 1,070 below the required total.
The shortfalls occurred even though the total number of service personnel required has fallen by almost 12,000 since 2005 when the forces were just 3,000 below strength.
Shadow defence secretary Liam Fox said the figures underlined the "retention crisis" facing the military.
"No amount of moving the goalposts can disguise the simple truth that the armed forces still face serious shortfalls," he said.
Liberal Democrat defence spokesman Nick Harvey said the figures showed the "terrible strain" imposed on the forces caused by fighting on two fronts in Iraq and Afghanistan.
Armed Forces Minister Bob Ainsworth said the Government had raised pay as well as introducing other financial incentives in an effort to improve retention rates.
"We face strong competition from other employers and our personnel are very much in demand owing to the skills and experience they acquire during service life," he said.
"At a time when the Armed Forces are heavily committed to operations, recruitment and retention of high quality people is more important than ever. We are determined to tackle these challenges."
