ROME (Reuters) - Former Prime Minister Silvio Berlusconi cast doubt on Tuesday on the prospects of the man he has indicated as a likely heir to head the divided center-right, saying he had spoiled his chances by clashing with an ally.

Berlusconi, 80, who underwent major heart surgery in June, had tapped Stefano Parisi, a former business chief and government adviser, as his candidate to relaunch Italy's splintered conservative forces.

Center-right parties are campaigning for a 'no' vote in a December referendum on Prime Minister Matteo Renzi's constitutional reform drive, hoping defeat will force him to resign and open the way for national elections.

Parisi staged a rally on Saturday against the reform, where he also took a swipe at Matteo Salvini, leader of the anti-immigrant Northern League, which has historically been close to Berlusconi's Forza Italia (Go Italy!) party.

"Parisi is trying to have a role within the center right, but given his conflict with Salvini, I don't think he can have this role," Berlusconi said on state radio RAI.

Polls show if Berlusconi's Forza Italia and the Northern League united with other conservatives behind one leader they could hope to compete with Renzi's center-left Democratic Party (PD) and the anti-establishment 5-Star Movement.

Parisi won Salvini's backing for a closely run - but ultimately unsuccessfully - campaign for mayor in Milan in June. But this week he branded the Northern League "Lepenists and populists", referring to the leader of France's National Front, Marine Le Pen.

"These are absolutely not definitive ruptures, they are personal disputes," said Berlusconi, who was expelled from the Senate and banned from office following a conviction for tax fraud in 2013.

Renzi said on Tuesday he had little desire to stay on as premier if Italians reject his constitutional changes.

All of the last 32 opinion polls by 11 different pollsters have shown Renzi losing next month.

(Reporting by Isla Binnie; Editing by Richard Balmforth)