What is the world's rarest passport? Rare document issued by order of crusading medieval knights

The Sovereign Military Order of Malta issues passports for its chief officers

Pope Francis poses with the Grand Master of the Sovereign Order of Malta, Fra' Matthew Festing (6th L) and delegates (Reuters)

With only one permanent passport issued at any time, the Sovereign Military Order of Malta is thought to have the most obscure passport of its kind in the world.

The Catholic order, which had its sovereignty recognised by Pope Paschal in 1113, is one of the oldest institutions in Christian civilisation.

It is a neutral, completely apolitical body that predominantly concerns itself with humanitarian issues - at the moment, for example, it is heavily involved in providing medical assistance to refugees crossing the Aegean Sea to europe.

The only permanent passport it issues is to its highest officer - the Grand Master.

Twelve individuals also hold temporary passports for the length of their five-year term. These positions include the traditional titles of Grand Commander (spiritual head) and the Grand Chancellor (Prime Minister) and the Grand Hospitaller (Minister of Health and Social Welfare).

The order also has diplomatic relations with 106 countries and the EU. Its permanent observer status at the UUN allows it access to crisis-hit regions where it can carry out humanitarian work.

Given its full title, the Sovereign Military Hospitaller Order of Saint John of Jerusalem of Rhodes and of Malta originated as the Crusader Knights Hospitaller in 1048.

Since losing the island of Malta in 1798, this military order's sovereign land is restricted to two extra-territorial properties in Rome, from where it issues its own postage stamps, currency and passports.

Currently this Catholic Order operates as a charitable organisation offering medical aid across the world.

It is made up of over 13,500 knights, dames and chaplains along with 80,000 permanent volunteers and 25,000 employees.

Countries that do not accept the Order's passports include the United Kingdom, the United States and New Zealand.

The current Grand Master is Fra' Matthew Festing, only the second Englishman to occupy the role.