Burnt oil was dumped on the protected cliffs in the limits of Birżebbuġa, known as Tal-Mara, home to a number of endemic plants.

Biodiversity expert Alfred Baldacchino said that the amount and spread indicated that more than one container of oil was used.

“This cannot be regarded as an accident or that the perpetrator/s simply wanted to dispose of the burnt oil. It would have been easier to push the containers over the edge, although even this would be illegal,” he said. “Such action does not occur in civilised countries,” he added.

The pollution was highlighted by environmental organisation Nature Trust, which received reports and photos of the dumping. The group said it was very concerned and pointed out that the cliffs, like other similar habitats, were home to indigenous and endemic plants species including the national plant Maltese rock centaury, or widnet il-baħar.

A spokesman for the Malta Environment and Planning Authority said that a team from its enforcement office was on site to investigate.

The cliffs are also home to the Maltese salt tree, the Maltese cliff orache and more common endemic plants such as the Maltese sea-lavender and the silvery ragwort, known as kromb il-baħar isfar.

Mr Baldacchino said the area was also the natural habitat of at least two breeding birds, the Cory’s shearwater and the Blue rock thrush, il­-merill – the Maltese national bird.

These plants and birds were the main reasons why the area was declared a special area of conservation and included in the EU Natura 2000 Network, he said.

“Unfortunately, the lack of monitoring, lack of communication, education, public awareness and enforcement can only serve as fertile ground for such actions,” Mr Baldacchino said.

Nature Trust appealed to the public to forward any information to the police, who were urged to investigate.