Italian coast guards found 25 people choked to death in the engine room of a tiny boat fleeing Libya crammed with 271 African refugees that landed yesterday on the island of Lampedusa.

The 15-metre boat was heavily overcrowded and survivors said they had been at sea for three days. Prosecutors said the victims, crowded in a space accessible only through a trap door, appeared to have died from asphyxiation.

Refugees cited in Italian news reports said the people in the engine room had tried to get out but were blocked by others because there was not enough space on deck, and probably died of intoxication from the engine fumes.

“We will carry out an autopsy to find the precise cause of death even though initial checks show that it was apparently due to asphyxiation. Police will question the other refugees,” local prosecutor Renato Di Natale told reporters.

Prosecutors opened an investigation into the deaths but have said they believe the refugees died in international waters as some of the corpses had already begun decomposing when they were pulled out by Lampedusa firemen.

“Given the state in which the corpses were found, they could have been dead for at least 48 hours,” a local doctor, Pietro Bartolo, told reporters.

The refugees did not initially mention the dead bodies and the grim discovery was made only when coast guards inspected the engine room below deck.

Lampedusa, which has a surface area of just 20 square kilometres, is Italy’s southernmost point and is closer to North Africa than to the Italian mainland.

It is now the biggest gateway for illegal immigration into the European Union following the arrival of tens of thousands from North Africa this year.