ROME – The security system shielding the Sistine Chapel from electronic eavesdroppers during this week’s conclave is supposed to be as infallible as a solemn papal pronouncement.

But it was under strain after journalists visiting the chapel, where cardinals will today begin the election of a new leader for the world’s Catholics, found that their cell phones worked in the building.

The Vatican has been anxious to demonstrate that, despite the ancient rituals of the conclave, it can counter high technology snooping by the modern press or curious intelligence services.

Officials said that “specialist technicians” had concealed sophisticated jamming equipment in the frescoed medieval chapel.

The equipment is designed to prevent listening devices, cell phones, or bugs from picking up any of the deliberations of the cardinals, who themselves take strict oaths of secrecy.

The Vatican’s chief spokesman, Joaquin Navarro-Valls, said before the tour by about 40 journalists that the Vatican’s own security team had installed the system.

He challenged them to test it themselves by attempting to use their phones in the chapel. Many of the group found that the reception on their phones was blocked.

But, embarrassingly for the Vatican, a number picked up signals. At least one journalist made a call.

Although it was not clear whether the jamming equipment was fully operational, the episode raised questions about the Vatican’s technical expertise.

At the last papal election in 1978, technology had not advanced much beyond the land-line telephone and the facsimile machine.