MILAN (Reuters) - European Commission President Jean-Claude Juncker hopes Italians will not reject reforms sponsored by Prime Minister Matteo Renzi when they vote in a referendum over changing the country's constitution, he told a newspaper on Sunday.

Despite Renzi's frequent attacks on the European Commission, Juncker told La Stampa that the Italian Prime Minister was carrying out the right reforms and added that the country's budget for next year was complying with EU rules.

"I don't know whether I would be useful to Renzi saying I would like to see 'Yes' win, I just say I would not like to see 'No' prevailing (in the referendum)," Juncker said.

"Italy is a great nation and Renzi has contributed to this, we have to admit this," he added.

Juncker's endorsement of Renzi's constitutional reform comes just a week before a Dec. 4 national referendum.

Italy's Prime Minister has promised to resign if he loses the ballot over his proposal to reduce the role of the Senate and transfer powers to central government from the regions.

Opinion polls conducted until a blackout period began last week showed the 'No' vote comfortably in the lead, prompting volatility in equity and government bond markets as investors fret about the political instability that could follow.

Renzi has attacked the Commission several times, saying it is run by bureaucrats, and he has asked for funds and support in dealing with an increasing flow of migrants arriving in the country from the Mediterranean.

Juncker said in the interview that the Commission would work to convince all European countries to share the burden with Rome.

"Italy is an essential part of Europe, should we lose it as Europe's architect, inspirer and artisan, it would not be the same," he said.

The 'No' camp supporters include the eurosceptic 5-Star Movement and the Northern League, other right-wing parties and some dissenters within Renzi's center-left Democratic Party.

(Reporting by Francesca Landini; Editing by Andrew Bolton)