Case opened after Marino makes public his expenses.

Rome prosectutors have opened an investigation into the expenses of the capital's embattled mayor Ignazio Marino, in the wake of a petition lodged by the anti-establishment MoVimento 5-Stelle (M5S) and the small right-wing party Fratelli d'Italia (FdI).

Currently there is nobody under investigation in the case which comes days after Marino took the unprecedented step of posting his official credit card expenses online, including his travel costs, from his election in 2013 until today. Marino says that each of his expenses has been itemised and published in a single file, to "facilitate consultation", and that his budget and spending are published in the interests of transparency.

The FdI claims that the credit limit of the mayor's credit card was increased from €10,000 to €50,000 in September 2013, three months after Marino took office.

The city council has responded by threatening to sue anyone who continues the "offensive, politically motivated and baseless" campaign in an attempt to smear the beleaguered mayor.

In recent days several of Marino's documented expenses have been queried: one by a priest, Fr Damiano Modena, who contradicts Marino's claim that he was a guest of the mayor at a dinner in May in the northern Italian city of Turin. The same priest also has doubts over whether he was Marino's guest at a meal in Rome in November 2013, following a presentation at the Capitoline Museums, however he acknowledges: "that was a long time ago..."

Another dubious expense relates to a "social Christmas meal" offered to representatives of the media at a restaurant in the Via Veneto area last December. This week the restaurateur, not knowing that he was being recorded by a hidden camera, said: “Marino was with his family. Plain and simple. But, I repeat, I will deny everything, eh."

The mayor's record of events has also been contradicted by the non-denominational charitable organisation Communità S. Egidio, which denies that its staff members were ever guests of Marino in October 2013, or at "any lunch or dinner since."

The mayor's office has stated that Marino spends a monthly average of €1,789 on diplomacy and PR-related expenses, using the city's credit card, and that in 2014, the city spent a total of just over €128,000 on such expenses. The mayor has also pointed out that his trips abroad have netted the city some €13 million in donations from foreign patrons to restore Rome's monuments.

Marino has been attacked by opposition politicians, the local media and comedians for months and particularly since returning from his contentious trip to Philadelphia where he travelled to the World Meeting of Families on 26 September, which was part of the pontiff's historic visit to America.

Pope Francis stated categorically on his return trip from the US that he had not invited Marino to the meeting, and the seemingly cool relations between the Vatican and the city of Rome are having an adverse effect on the planning for the Holy Jubilee year due to open on 8 December.