Plans to drill for oil off the Spanish holiday island of Ibiza have met with fierce opposition from locals who fear the white sand beaches and marine life will be damaged.

"Everyone is against it here," said Carlos Bravo of the Blue Sea Alliance, an umbrella organisation of 50 groups including environmentalists, hoteliers and unions, seeking to halt oil exploration off Ibiza and Formentera, a neighbouring island that is only accessible by boat.

More than 10,000 people, some carrying signs saying "The Mediterranean is not for sale", marched in February through the streets of the capital Ibiza Town against the planned exploration.

Their campaign has the backing of celebrities like US socialite Paris Hilton and British model Kate Moss who are regular visitors to Ibiza, one of Europe's top tourist destinations which is famous for its nightclubs and turquoise waters.

"Ibiza is in danger. It is not a myth, not a metaphor and not an exaggeration. The island is facing an ecological disaster," Hilton wrote on her Instagram page earlier this year.

Moss has signed a petition urging the Spanish government not to allow the oil prospecting to go ahead, and posted a photo of herself holding a sign that read "Ibiza Says No" to her 32.8 million Twitter followers.

The project dates back to 2010 when British firm Cairn Energy obtained four exploration licences for the waters around the Balearic Islands, one of which is Ibiza.

The company now plans to use seismic imaging, which bounces sound waves off the ocean floor, to map pockets of underground oil in the region before it applies for government permits to start drilling.

Cairn Energy said it will carry out the seismic imaging over 2,400 square kilometres (930 square miles), an area roughly the size of Luxembourg, 53 kilometres (32 miles) from Ibiza.

"The company is currently at the very early stages of assessing whether to explore for hydrocarbons," it said in a statement, before adding that the seismic imaging will take place "at a time when any impact will be at a minimum, most likely in the winter months".

- Noise threatens marine life -

Environmentalists warn the sonic shocks used in seismic imaging are a threat to marine life.

The deep waters around the Balearic Islands are home to the endangered bluefin tuna, the striped dolphin, the long-finned pilot whale and sperm whales.

"Cetaceans are very sensible to noise. This will interfere with their capacity to communicate and search for food," said Txema Brotons, the president of Tursiops, a research and conservation group based in the Balearic Islands.

The seismic imaging will use airguns that produce a loud sound every 10 seconds, round-the-clock, for 75 days, said Bravo of the Blue Sea Alliance.

"They generate a huge noise level, 10,000 to 100,000 times louder than the engine of an plane, which seriously harms fish, cetaceans, turtles and invertebrates," said Bravo.

Local fishermen fear the seismic imaging will ruin their catches.

"Cetaceans, fish and their larvae... they are all going to leave," said Pere Valera, the head of Ibiza's main fishermen's group.

"We have seen what has happened elsewhere, there was a 70 percent reduction in catches after seismic imaging."

- 'Islands' oil is tourism' -

Opponents also fear that oil exploration will hurt tourism in Ibiza, the main source of revenues for the island which is an UNESCO World Heritage Site.

"The Balearic Islands' oil is tourism," said the head of the islands' regional government, Jose Ramon Bauza, who belongs to the conservative Popular Party in power at Spain's national level.

Of all the advanced economies of the OECD grouping, Spain is the most reliant on energy imports, which account for 99.9 percent of its oil and gas.

The Spanish government is looking to reduce this dependence by developing Spain's own energy production.

The Spanish oil industry could create 250,000 jobs and account for 4.3 percent of gross domestic product within 20 years, according to a report published last month by consulting firm Deloitte.

The report is based on an estimate that Spain has reserves of two billion barrels of oil.

Environmentalists are discouraged by the example set by Spain's Canary Islands, where seismic imaging has already taken place and oil drilling is set to begin later this year despite protests by island residents.