Iranian ex-president Akbar Hashemi Rafsanjani backed Hassan Rowhani for Friday's presidential election, saying the former nuclear negotiator who champions better international ties is "more suitable" for the post.

"I will vote for Dr Rowhani, who entered the race after consulting me," Rafsanjani said in remarks reported by the ILNA news agency on Tuesday.

"I consider him to be more suitable to steer the executive branch," he said referring to the slate of five other hopefuls, most of whom are conservatives.

It was the second major endorsement of the day for Rowhani, following that of reformist former president Mohammad Khatami, under whom he headed Iran's negotiations with the major powers on its nuclear programme.

Rowhani is facing stiff competition from the conservative camp, which has failed to field a single candidate.

Tehran mayor Mohammad Baqer Qalibaf, top nuclear negotiator Saeed Jalili, ex-foreign minister Ali Akbar Velayati and former Revolutionary Guards commander Mohsen Rezai remain in the race to succeed President Mahmoud Ahmadinejad.

Mohammad Reza Aref, first vice president under Khatami and the only out and out reformist contender, dropped out of the competition on Tuesday.

Rafsanjani's endorsement comes after his own candidacy was rejected last month by the hardline electoral watchdog, the Guardians Council, whose members are appointed by supreme leader Ayatollah Ali Khamenei, directly or indirectly.

According to the ILNA report, Rafsanjani said his disqualification came after "a high-ranking security official, against the norms and law, attended the vetting meeting in the council."

"The official said: 'Rafsanjani's presence in the election could lead to his decisive victory in a landslide'," the ex-president said of the meeting, without naming anyone.

"The official then convinced the council to disqualify me on excuses of frailty," he said.