Two researchers have died after an experimental hybrid car exploded during a test run in Italy, the country's National Research Council said on Thursday.

The Volkswagen Polo TDI exploded on the Naples ring road on Friday last week.

The fire brigade is investigating the contents of two gas cylinders on board the vehicle, which are believed to have caused the blast.

Trainee Fulvio Filace, 25, died on Wednesday night, the NRC said. He had been put in an induced coma after suffering third-degree burns over 70 percent of his body.

Researcher Maria Vittoria Prati, 66, a specialist in the study of emissions and the use of alternative fuels, died on Monday.

She had been driving the car while Filace was in the passenger seat.

Naples public prosecutor's office has opened a homicide and fire investigation, and has seized a prototype of the vehicle, according to Italy's AGI news agency.

The NRC expressed "sorrow" and "dismay", adding that it was launching an internal probe.

"The loss of two lives, all the more so in such dramatic circumstances, has a profound impact on the scientific community throughout Italy," it said.

The car was being used as part of a European research project. It aimed to test an electric motor with batteries powered by solar panels on vehicles with internal combustion engines, according to AGI.