A rock structure in the form of an arch which had featured in countless Malta tourism brochures collapsed into the sea on Wednesday in what Prime Minister Joseph Muscat described as a "heartbreaking event".

Geologists had long warned that the structure, on the north-western coast of the small island of Gozo, was eroding fast and the authorities had banned visitors from walking on top of it.

The structure collapsed as Malta was hit by rough seas and stormy weather.

Gozo resident Roger Chessell went to the coastline in the morning to take pictures.

"There was a big raging sea beneath the Azure Window," he told the Times of Malta newspaper. "Suddenly, the arch collapsed into the sea with a loud whoomph, throwing up a huge spray."

Prime Minister Muscat said in a Tweet that the famous Mediterranean landmark had always faced destruction because of natural corrosion. "That sad day has arrived," he wrote.The stunning geography of the area was created thousands of years ago when two limestone caves collapsed.

The arch, among Malta's biggest tourist attractions, was immortalizedin several films and in an episode of the popular television series "Game of Thrones."

In January 2016 rough seas had exposed the fragility of the site, with waves breaking off a large slab at the base of one of its cliffs.

In January, the authorities imposed a fine for anybody walking over the rock arch, to try to stop its disintegration.

The Azure Window, located on Malta's sister island of Gozo, is also overwhelmingly popular among divers.

Rough winds hit the Mediterranean island on Tuesday and Wednesday, even halting the ferry service between Malta and Gozo for a number of ours.