PRIVATE security teams patrol the decks of around 40% of large vessels in the “high-risk area” that stretches from the Persian Gulf to the Seychelles in the south and the Maldives in the east. When pirates attack, these armed guards respond with flares or warning shots. This usually scares off assailants (or sends them in search of easier prey). If it fails, they fire at an attacking boat's engine, before finally turning their sights on the pirates. No ship carrying armed guards has so far been hijacked.

Most of the companies providing these guards are British, typically started by entrepreneurial former special-forces types. A four-man team can charge $45,000 for safe passage through the high-risk area. The cost to shipowners is partly offset by savings on insurance.

The idea may seem simple but its legal framework is not. Under the United Nations Convention on the Law of the Sea a ship's crew, including guards, must abide by the home laws of a vessel's flag state. But these vessels ply international waters, meaning that regulation is scant. An array of standards created since 2009 suggests good practice for private security teams, but none is legally binding.

Spurred on by the International Maritime Organization (which will debate the issue at a meeting next month), governments are now trying to write rules for armed guards at sea, such as how they buy and store the lethal tools of their trade. Britain wants a voluntary set of rules in place by the end of 2012, detailing the acceptable use of deadly force and systems for company auditing and accountability. It may suggest and define a “proportional” response to pirate attacks, along with approved weapon types and standards of training. Other countries are making moves too. American law now allows for the self defence of US-flagged ships within tight rules of engagement. India also allows armed guards; Greece is considering a similar step. The Japanese government is pondering a change to its strict laws, which prohibit civilian armed guards on ships.

The United Arab Emirates will this year start allowing armed international teams into its ports. At present most teams use Sri Lanka, Oman or Djibouti for weapons storage between jobs. They run the risk of prosecution if they carry arms in the territorial waters of Yemen and other states. John Bennett, of the Florida-based Maritime Protective Services says some firms play safe by throwing their guns overboard before heading home.