Defeated Iranian presidential candidate Mehdi Karroubi on Sunday demanded an independent panel to probe irregularities in the disputed presidential election.

"If an independent panel is set up by the Guardians Council with full responsibility to investigate all aspects of the election, I will welcome it and later nominate my representative," the reformist Karroubi wrote in a letter to the council which was published in his newspaper 'Etemad Melli'.

The Guardians Council, Iran's electoral watchdog, has set up a panel to conduct a partial recount of ballot boxes from the June 12 election that returned Mahmoud Ahmadinejad to power, and to prepare a report on the vote.

But the main opposition leader Mir Hossein Mousavi, who was Ahmadinejad's strongest rival, rejected the panel on Saturday.

The other defeated candidate, the conservative Mohsen Rezai, has agreed to be part of the council panel if Mousavi and Karroubi also agree to nominate representatives to the body.