The British Ministry of Defense had an unusual security breach recently when an email containing the promotions of noncommissioned officers, some of whom serve in special missions units, was accidentally distributed across the government.

Among the regular promotions of conventional troops were the names of commandos with the Special Air Service, Special Boat Service, and Special Reconnaissance Regiment, as well as the Special Forces Support Group.

Some of those named serve in an elite, classified outfit known as Special Air Service, E Squadron — or “the Increment.”

A secret unit within a secret world, E Squadron works for the British intelligence services in high-risk operations overseas.

When Special Operations Meets Intelligence

The British military is a pioneer in modern special operations forces, creating the first modern units during World War II.

Since then, British special operations units have led the way, establishing doctrine and tactics, techniques, and procedures that are now in common use across the world, including in the U.S.

The SAS, SBS, and SRR are the British military’s three main Tier One units.

The first two focus on direct-action, counterterrorism, and hostage-rescue operations and are the British equivalents of the U.S.’s Delta Force and SEAL Team 6, with which the British units work closely and even exchange operators.

The SRR specializes in gathering human intelligence and signals intelligence and in the operational preparation of the battlefield.

The British Ministry of Defense is working on modernizing the country’s special operations units to reflect lessons learned from the past two decades of combat experience in Afghanistan, Iraq, Syria, and Africa.

Interestingly, the British are basing their modernization process on U.S. special-operations forces and their evolution over the past 40 years. The teacher has slowly become the pupil.

But in some instances, intelligence services need the specialized skills and training of commandos. That is where E Squadron comes in.

The British intelligence apparatus is composed of three agencies.

The Secret Intelligence Service, known as “MI6,” is the British equivalent of the CIA and specializes in foreign intelligence gathering and covert action.

The British also have the Security Service, better known as “MI5,” that conducts domestic counterintelligence and is the equivalent of the FBI.

Finally, the Government Communications Headquarters, specializing in signals intelligence, is the equivalent of the NSA.

The Increment

Although E Squadron’s mission sets are classified, open-source information suggests that the unit provides manpower to MI6 operations abroad.

Their missions can include close-protection details, in which they act essentially as bodyguards, as well as extracting assets from, conducting special reconnaissance of, and supporting covert action in denied environments, such as Russia, Iran, China, or North Korea.

E Squadron operators work undercover, using aliases and backstories.

The British government played a big part in the campaign to overthrow the Libyan dictator Muammar Gaddafi in 2011. Not only was MI6 deeply involved, but British special operations units, including E Squadron operators, also saw limited action in the country while conducting special reconnaissance and close protection.

But an E Squadron mission was compromised in Libya, resulting in the capture of several SAS members, who were later released, and the embarrassment of the British government.

Although E Squadron recruited mainly from the SAS in the past, it targets candidates from across the British Tier One community, with SBS and SRR operators also joining the shadowy outfit.

Besides a special-missions-unit background, E Squadron recruits candidates based on their ethnic background, a reflection of the U.K.’s colonial past and the fact that people from many foreign countries, such as Fiji, Malta, and Jamaica, can join the British armed forces.

The British military wields that openness as a strategic advantage. British citizens of Pakistani, Indian, Yemeni, Syrian, or Nigerian backgrounds can join and work their way up to the SAS, SBS, or SRR.

If they perform well in a demanding environment, complete several overseas deployments, and shine beyond the direct-action and counterterrorism aspects of the job — such as reliably working on their own to conduct low-visibility prep work — they can be assessed for service in E Squadron.

They would have to pass an additional selection and a demanding training course that would emphasize intelligence tradecraft more than additional special-operations skills.

“You don’t know a lot about them. There’s a veil of secrecy, and guys who end up there just disappear,” a former SBS commando who spoke to Insider said of E Squadron members.

“But honestly that also happens in the Regiment and in my own old unit and also at the SRR,” the former commando added. “Guys who you might be close mates with will go on an assignment, and you won’t know where they’re or what they’re doing. It’s standard and part of the job. But it’s on a completely different level” with E Squadron.

Maintaining the covert nature of the Increment’s missions demands absolute secrecy, meaning an email flap could be a potential disaster for operational security, burning commandos and sidelining them from future operations abroad.

This article was written by Stavros Atlamazoglou and originally published on the Insider.