The Army celebrating the Queen's birthday in 2017 (Picture: MOD).
Greater numbers of foreign nationals are have been made eligible to join the armed forces as ministers try to improve recruitment.
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.
In October, the captain of Britain's biggest and most powerful warship said it would be "lovely if we had another 10,000 people in the Navy".
However, HMS Queen Elizabeth Captain Jerry Kyd said the Navy was "OK" and "getting back into balance".
Sir Mark said that 22% of the officers and 20% of other ranks for the Royal Navy had been recruited over the three months.
100% of officers and 10% of other ranks had been recruited for the Royal Marines over the same period.
Royal Air Force
Some areas of the armed forces are reportedly seeing improvements, with the RAF recording its best recruiting year in a decade.
In October, Chief of the Air Staff Air Chief Marshal Sir Stephen Hillier said recruitment was "strong" and they were hitting "about 100%" of their recruiting targets.
Sir Mark said the RAF had recruited 56% of officers recruits and 48% of other ranks over the three months to August.
The NAO report said there was a need for 800 pilots, especially in the RAF.