The Royal Marines are the amphibious assault specialists of the United Kingdom's Royal Navy and are trained to be deployed quickly in any terrain. The Marines are a disciplined fighting force and expect clean-cut warriors, especially during occasions in which soldiers march on parade. Anything that distracts from the decorum of the Marines is discouraged. This can include tattoos. But the Royal Marines won't turn away any applicant because of some ink, and the force has guides for which tattoos are allowed and which are too much to handle.
The Royal Marines does not allow tattoos that are visible on the head or hands. Tattoos that are drawn low on the neck may be allowed, provided they cannot be seen above the collar of the formal uniform (This is difficult to get away with, however, because the top button of the uniform's collar is worn unfastened.) The uniform provides a wearable guideline for what's acceptable: if it's visible while you're in uniform, it's against regulations.
The Royal Marines do not allow tattoos that can clearly be interpreted as offensive. Tattoos that are racist, sexist, lewd, crude, obscene or intimidating are barred for their potential to be repugnant to the public and divisive in an environment that requires teamwork. Examples include swastikas, cartoon gore, nudity or genitalia, and crudely drawn homemade tattoos.
Tattoos that are inoffensive and covered by the formal Marines uniform may still be barred if they are taking over all the prime real estate on the skin. Examples include tattoos that cover the entire back, or large portions of it, or tattoos that cover the entire arm (known as a "sleeve"). Marines are allowed to display forearm tattoos while on duty, but they must remain "presentable," at their commanding officer's discretion.
Discretion of the Commanding Officer
Commanding and recruiting officers have the right to decide if a tattoo is inappropriate for display. If it's not, he can order a Marine to cover up his tattoo in public and, in some cases, the Marine must have the tattoo removed at his own expense. This isn't just for the benefit of a clean public image: the Royal Marines guidelines warn against tattoos that can draw unwelcome attention during special military operations.