Italian oil major Eni has delayed the planned $3.4 billion sale of its domestic retail business, sources said, as political uncertainty caused by an upcoming referendum on democratic reform hinders major deals across the country.

The state-controlled company had hoped to start the sale process late this year but has rescheduled it for 2017, betting that the fate of Prime Minister Matteo Renzi and his government will become clearer after the vote, according to two sources close to the matter.

Renzi has repeatedly vowed to quit if he loses the referendum. A new prime minister could lead to the appointment of new executives at Eni and a change of strategy, the sources said.

The vote, to be held in late November or early December, calls for a slimming-down of Italy's political system, the centre-piece of Renzi's plan to speed up the legislative process.