Italy's transport authority late on Thursday unveiled changes to the way tariffs applied by motorway concession holders are calculated, effectively lowering returns that companies can expect to reap on their investments.

The two sources, speaking on condition of anonymity, said the changes could not come into force until the transport ministry had revised the existing concession contracts.

One source said this could not be done without holding talks with concessionaires and they hoped to sit down with the ministry staff.

However, a transport ministry source said the latest changes would automatically come into force from January 2020.

In a post on Facebook, Transport Minister Danilo Toninelli said late on Friday that in the near future tariffs would "not suddenly skyrocket and could even decrease".

The new tariff scheme on Friday drove down by 4.31% shares in Atlantia, the infrastructure group that controls Italy's biggest motorway operator Autostrade per l'Italia.

The changes were hailed as "the start of a revolution" by 5 Star's Toninelli, who said any criticism showed the government was on the right track.

Italy's industry lobby Confindustria said it was very worried that the new system would fuel conflicts and uncertainty, putting a brake on investments by concessionaires.

"We must absolutely avoid getting into a legal dispute that could put a stop to toll-road investments, which would be damaging and unacceptable for recovery prospects and our country's international image," Confindustria said.

Toninelli tried to quell industry's fears by saying on Facebook that the motorway concessionaires that invest will be rewarded.

(This story has been refiled to amend tense in para 6 to 'would' from 'will')

(Reporting by Valentina Za and Stefano Bernabei, writing by Giselda Vagnoni, editing by Silvia Aloisi, William Maclean)