Britain's armed forces are significantly understaffed in critical areas such as intelligence and engineering, Whitehall's spending watchdog, the National Audit Office, has warned.
A report published by the NAO states that as of January this year the country's full time military was operating at 5.7% below the current target.
With more than 8,200 regulars needed to meet the requirement, the NAO highlighted how this is the "largest gap in a decade".
The NAO stressed that alongside the gap in personnel numbers across the Army, Royal Navy and Royal Air Force, there are "much larger shortfalls in critical skills".
With 102 "pinch-point" trades, the watchdog said there are not enough trained regulars to carry out operational tasks without cancelling leave or training within these areas.
According to the NAO most of these so-called pinch-points were in six areas including engineering, intelligence, logistics, pilots, communications and medical.
This includes a lack of some 2,400 engineers - with the largest among Royal Navy weapons engineers - a shortfall of 700 intelligence analysts, and a need for 800 pilots, especially in the RAF.
Labour have laid the blame for the shortages squarely on the shoulders of the Government.
Nia Griffith, Labour's Shadow Defence Secretary, said: “First of all they’ve slashed the armed forces. They’ve really damaged morale.
"But most importantly of all they’re not treating our armed forces properly."
"They’re not paying them properly and there’s greater dissatisfaction now with pay and conditions of service life than there has been ever so it’s really worrying.”
With the impact of the shortfalls becoming "more severe over the past year", just six are expected to be resolved over the coming five years, with 23 becoming worse, the NAO said.
The report said the reliance by the Ministry of Defence (MOD) on a base-fed workforce model, where young recruits in lower ranks are developed and trained over time, has not enabled capability gaps to be closed quickly enough.
With constantly evolving threats facing the UK, the NAO said the "increasing risk of cyber and electronic attacks will change the capabilities and skills that the armed forces will need in future".
"The new demands will add to the pressure to increase capability in some trades that already have shortfalls," the report added.
Meg Hillier, chairwoman of the Public Accounts Committee, said:
"In these uncertain times, it is more critical than ever that Britain has a well-staffed armed forces with the technical know-how to handle threats to national security.
"But the NAO report shows that the armed forces are woefully below compliment, especially in crucial areas like intelligence and engineering."
With a 24% shortfall in the number of regulars recruited to the armed forces between 2016 and 2017, the number of regulars leaving the military voluntarily is on the up, the NAO said.
It said the percentage of those choosing to leave the armed forces has increased from 3.8% a year in March 2010 to 5.6% in December 2017.
In a bid to tackle the shortfalls, which the NAO said "result mainly from recruitment and retention problems", the MoD spent more than £664 million on these areas in the last five years.
But Ms Hillier said the MOD "needs to take a long hard look at its current approach".
"Without more innovative methods to attract and retain staff, the UK risks continuing with big gaps in capability and overstretching already hard working and crucial service personnel," she said.
NAO chief Sir Amyas Morse said ensuring the armed forces have the right number of skilled personnel is "not a new challenge" facing the MOD.
"But given the complexity and development of new, modern-world threats, it is a challenge that will only continue to grow," he said.
"The department needs to fundamentally change its approach to develop skilled personnel and address the long-established shortfalls that persist."
An MOD Spokesman said recruiting and retaining talent is a "top priority" and there are a range of schemes used to attract and keep skilled personnel. He said:
"The military has enough personnel to meet all its operational requirements, including being active on 25 operations in 30 countries throughout the world
"In the past year we have recruited over 13,000 people into the armed forces."