ROME (Reuters) - Prime Minister Matteo Renzi's reform agenda for Italy suffered a setback on Thursday when 13 senators from his centre-left Democratic Party (PD) withdrew their backing in protest against his moves for constitutional change.

The 39-year-old former mayor of Florence will not be assured of a majority in the Senate without the 13 defectors and their absence will make it extremely difficult for him to push through legislation.

Mr Renzi's plan to abolish the Senate as an elected chamber in order to streamline the passage of legislation, set out when he took office in February, has been held up in the Senate Constitutional Affairs Committee. This week, Mr Renzi removed two PD opponents of the plan from the committee.

In a shock response on Thursday, 13 PD senators said they were "suspending themselves" from the party, accused Mr Renzi of authoritarian tactics at odds with the constitution and said they wanted an open discussion with the Prime Minister.

Mr Giuseppe Civati, a PD deputy frequently critical of Mr Renzi, said his removal of the senators from the committee was "a political error" and rebuked him for trying to "eliminate dissent" in the party.

The senators did not say they would leave the PD itself and their action was seen more of a warning shot to Renzi than an immediate threat to his government's survival.

A crushing victory by the PD in elections to the European Parliament last month was widely seen as a personal triumph for Mr Renzi, but despite his personal popularity he has so far failed to make much headway with a raft of promised reforms.

One of them targets the Senate, which Mr Renzi says is a drag on the legislative process and a financial drain on resources.

His blueprint for an overhaul of the electoral law, agreed in January with centre-right leader Silvio Berlusconi, has made no progress in Parliament and Berlusconi frequently suggests he may pull his support.

Mr Renzi, who has been on a tour of Asia this week, said on Wednesday the European elections had shown Italians back him and he was determined to pursue reforms despite political resistance. "Considering that votes count for more than vetoes, we will be carrying on with our heads high," he told reporters in China.