France is recalling its ambassador to Italy for talks after what it said was a series of “unfounded attacks and outlandish claims” by Italian leaders.
The French foreign ministry said it was a situation “unprecedented” since the end of World War Two.
On Wednesday, France warned Italian Deputy PM Luigi Di Maio not to interfere after he met French “yellow-vest” protesters near Paris.
The two governments have also clashed over issues such as immigration.
The latest spat began after Mr Di Maio, leader of the populist Five Star Movement, met two leaders of the anti-government movement on Tuesday.
He posted a picture of himself on Twitter with a yellow-vest leader and with Ingrid Levavasseur, who is heading a yellow-vest (gilets jaunes) list for elections to the European Parliament in May.
What has France said?
“For several months France has been the subject of repeated accusations, unfounded attacks and outlandish claims,” the foreign ministry said on Thursday.
“The most recent interferences constitute an additional and unacceptable provocation. They violate the respect that is owed to democratic choices made by a nation which is a friend and an ally. To disagree is one thing, to exploit a relationship for electoral aims is another.”
On Wednesday, the ministry called Mr Di Maio’s visit a “new provocation” that was “unacceptable between neighbouring countries and partners at the heart of the EU”.
The BBC’s Hugh Schofield in Paris says the row represents a new low in the fast deteriorating relationship between Paris and Rome.
What is the background?
Relations between France and Italy have been tense since the Five Star Movement (M5S) and right-wing League parties came to power in Italy in June 2018.
In January, France summoned Italy’s ambassador after Mr Di Maio said Paris had “never stopped colonising tens of African states”.
Italian leaders have clashed with France on issues such as migration protests and culture.
Who are the ‘gilets jaunes’?
The “gilets jaunes” protests against fuel tax hikes began last November, saying the measure hurt those who live in remote areas of France and depend on cars.
They derive their name from the high-visibility vests they wear – and which French motorists are required by law to carry in their vehicles.
But since their first marches – and the government’s subsequent U-turn on fuel taxes – their demands have expanded to boosting people’s purchasing power and allowing popular referendums.