Mr Karoubi's son says his father still has doubts about the vote, but accepts the results following Supreme Leader Ayatollah Ali Khamenei's endorsement.
Opposition activists are preparing for a new round of anti-Ahmadinejad protests on Revolution Day next month.
According to Iran's Fars news agency, Hossein Karoubi quoted his father as saying: "I am still of the same belief that the election was unhealthy and massively rigged.
"But since the [supreme] leader [Ayatollah Ali Khamenei] endorsed [Mr Ahmadinejad's victory], I believe that he is the head of the government, meaning he is the president.">>>
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