VATICAN CITY, June 26 (Reuters) - Pope Francis, a vocal campaigner for action against climate change and on environmental protection, on Wednesday ordered the construction of a solar farm to cover the energy needs of the Vatican City State.

The mini-state, based in Rome, hosts the headquarters of the global Catholic Church and comprises St Peter's Basilica. It is the world's smallest country, measuring just 0.44 square kilometers.

Expressing his wishes in a Motu Proprio, a personal papal edict, the 87-year-old pontiff said the solar farm should be built outside Vatican walls, in Santa Maria Galeria in the northwestern outskirts of Rome.

The area belongs to the Vatican and already hosts a broadcasting center for Vatican Radio, which will also be powered by the new solar farm. The papal edict did not specify how big the farm will be or when it would be ready.

"We must transition towards a sustainable development model that reduces greenhouse emissions into the atmosphere, aiming towards climate neutrality," Francis said in the two-page document.

Francis issued a warning on the dangers and challenges of climate change and on the need to reduce the use of fossil fuels in his landmark "Laudato Si" (Praised Be) encyclical from 2015.

In it, he warned that the planet was "beginning to look more and more like an immense pile of filth."

He followed it up last year with the "Laudate Deum" (Praise God) Apostolic Exhortation, in which he called on climate change deniers and foot-dragging politicians to have a change of heart, saying they cannot gloss over human causes or deride science while the planet "may be nearing the breaking point."

The pope gave the task of building the solar farm to the cardinal who acts as governor of the Vatican City State and to the head of APSA, the department that manages the Vatican's portfolio and is usually described as its central bank.

(Reporting by Alvise Armellini; Editing by Hani Richter)