Eta plot to blow up hundreds of Britons on a car ferry

The Pont-Aven, which sails twice a week between Plymouth and the northern Spanish port of Santander, was one of three possible targets, say officials.

It is believed the terrorists planned to use a car bomb in the attack.

If it had exploded at sea it would have been catastrophic for the vessel and its 2,400 passengers and 183 crew.

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Eta separatists planned to use a car bomb to blow up this ferry from Plymouth to Santander

The plot was hatched by Eta, which has been fighting for three decades for an independent Basque region encompassing parts of northern Spain and southern France.

The group had agreed a ceasefire which ended just over a month ago, and Spanish officials have warned of an 'imminent major attack'.

Two vehicle bombs planted by Eta have recently been intercepted on their way to Spain, one from Portugal and one from France.

The plot to murder British tourists was discovered after a suspected Eta terrorist was arrested at the Santander bus station on Tuesday.

Aritz Arginzoniz Zubiarre was caught carrying a detonator commonly used by Eta for car bombs, a pistol, and false identity documents.

He had been wanted by Spanish police since March, when they discovered explosives and detonators in raids in the Basque region.

The 22-year- old told the police who arrested him he was a member of Eta. The Spanish newspaper El Pais said he had details of at least two potential

targets with him when he was arrested, one a courthouse in Santander and the other the ferry bound for Plymouth.

It is believed he had been staying at a campsite 27 miles away with his girlfriend, Saioa Sanchez Iturregi. Police were still looking for her last night.

It is thought Arginzoniz Zubiarre was waiting for explosives and a car to be delivered at the time of his arrest.

Spanish interior minister Alfredo Perez Rubalcaba said: 'He was going to strike against a public building in the next few days, presumably in a car park. Eta is going to keep on trying to attack, but we are going to keep on trying to make sure they don't succeed.'

A bomb attack Eta had planned for the end of last week was thwarted when French authorities intercepted a van packed with explosives.

Last month British tourists were warned an Eta terror cell was preparing an attack on the Costa del Sol. Spanish police asked holidaymakers in the region - visited by 3.6million Britons a year - to be vigilant.

A Spanish counter-terrorism official said: 'Eta used the ceasefire to move terrorists, explosives and vehicles all over Spain. We believe they could carry out a summer campaign against tourists similar to previous years.'

In July 2003, Eta's suitcase bombs injured 13 people in Alicante and Benidorm and in June 2002 five Britons were injured by a car bomb in nearby Fuengirola.

Eta has attempted attacks at sea before. Several years ago it planned to load a van bomb on to a ferry sailing from Valencia to the Balearic Islands, but the van broke down and the plot was abandoned.

Eta, the Spanish acronym for Basque Homeland and Freedom, was founded in 1968 and has killed 810 people so far.