In January this year, the Army, Navy and RAF had 137,300 trained regulars, 8,200 below their current requirement, according to a report by the public spending watchdog.
The 5.7% shortfall was the largest for a decade, said the National Audit Office (NAO), which warned there were "much larger" shortfalls in the number of regulars with critical skills, like pilots, engineers and intelligence analysts.
Here is a look at the recruitment pressures facing Britain's military forces.
The size of the regular Army plummeted from 159,100 in 1980 to around 83,600 in 2017, according to Ministry of Defence statistics.
Between 2010, when 108,920 were serving, and 2017 there was a steady decline in the number of full-time personnel.
The Commons Defence Committee was told in October that the Army currently has 77,000 fully trained troops compared with a target of 82,500.
A report commissioned by Downing Street, published in 2017, found the Army was running at more than 30% below its annual recruitment target.
Outsourcing giant Capita has had the contract for Army recruiting since 2012.
The MOD disclosed in October that Capita managed to bring in fewer than one in 10 of the recruits the Army needs for 2018-19 in the first quarter of the year.
Meanwhile, Deputy Chief of the Defence Staff Sir Mark Poffley said that, in the three months to August, the Army had signed up just 10% of the officer recruits and 7% of the other ranks it needed for 2018-19.
Capita said regular soldier applications were at five-year high and the number of overall eligible applications had increased by 71% between 2013 and 2018.
More than 32,000 people make up the full-time force of the Royal Navy and Royal Marines.