RAF Brize Norton was protected by little more than a six-foot wooden fence when Palestine Action activists breached its defences on Friday morning.
Images of the Oxfordshire base, which has acted as one of Britain's most strategically important military airfields for nearly 80 years, show a large section of the eight-mile perimeter was secured only by fencing similar to that used in a home garden.
The vulnerability allowed two protesters to penetrate the base, which serves as the UK's hub for strategic air transport and refuelling operations and tag an aircraft with red paint.
The airfield regularly hosts members of the Royal Family and senior politicians on international flights, making the apparent security shortcomings particularly concerning.
Home Secretary Yvette Cooper will move to proscribe Palestine Action in the coming weeks, effectively designating the group as a terrorist organisation, it is understood.
Cooper is preparing a written statement before Parliament on Monday which, if passed, will make membership of the group illegal.
During the incident, activists sprayed red paint on two military aircraft, in what Sir Keir Starmer has branded "a disgraceful act of vandalism".
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A long stretch of fence is not covered by barbed wire and stands at just 6ft off the ground
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Palestine Action entered the base and spray-painted the engine of a Airbus Voyager aircraft
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South East counter terrorism police confirmed its specialist officers were investigating alongside Thames Valley Police and the Ministry of Defence.
The upcoming proscription marks a significant escalation in the Government's response to Palestine Action's targeting of military sites.
Footage posted online by Palestine Action showed two people entering the base in darkness, with one riding an electric scooter up to an Airbus Voyager aircraft.
They then used "repurposed fire extinguishers to spray red paint into the turbine engines" and employed crowbars to inflict additional damage.
An aerial shot shows where the plane was tagged
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An aerial shot of the airfield shows several of the planes were on the tarmac at the time
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The incident occurred in the early hours of Friday morning, with Palestine Action claiming its members "escaped undetected" after the attack.
The group stated that flights leave daily from Brize Norton to RAF Akrotiri in Cyprus, alleging that "from Cyprus, British planes collect intelligence, refuel fighter jets and transport weapons to commit genocide in Gaza".
RAF sources indicated the damage appeared superficial, though all aircraft would undergo full engineering checks.
Justin Bronk, a senior research fellow for military airpower at think tank Rusi, described the breach as "a fairly catastrophic failure of force protection" requiring urgent attention.
Barbed wire on top of the fence stops in front of the runway
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"The challenge of defending air bases from unconventional attacks is a cross-departmental problem, but one that needs to be urgently looked at," he told The i Paper.
He emphasised the need for greater cooperation between the Armed Forces, police and intelligence services.
"Questions will be asked. I would suggest it's probably much more important to look at the systemic failures and how to address them, rather than to try and look for a scapegoat," Bronk said.
Greg Bagwell, a former RAF Air Marshal, described the runway incursion as another "wake-up call" for domestic security on military sites.
Other parts of the fence are more fotified, with barbed wire and multiple layers of fencing
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RAF Brize Norton
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One former guard at an overseas RAF base told The i Paper that security was "lax and hugely out of date", citing an incident where a local farmer had been given a key to a fence next to the runway "in case he wanted to herd his livestock through".
RAF insiders acknowledged that successive funding cuts and shrinking Armed Forces staff numbers meant there were insufficient personnel to guard sites with "huge perimeters".
Military teams typically rely on intelligence to pre-empt attacks, increasing patrols according to threat levels, but insiders admitted they lacked adequate knowledge of the protesters' plans.
This month's Strategic Defence Review had already highlighted Brize Norton as needing "investment and improvement", particularly in contingency planning to make RAF logistic support more resilient to disruption.
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