The head of Iran's atomic agency says power generation from its first nuclear plant will probably begin in January - two months later than announced.
Ali Akbar Salehi said the apparent delay at the Bushehr reactor had nothing to do with the Stuxnet computer worm, but gave no other explanation.
The Russian-built plant was inaugurated in August after several years of delay. It will be internationally supervised.
It is not seen by analysts as posing a significant proliferation risk.
The process of placing fuel rods at the Bushehr facility would be completed in early November, Mr Salehi, who heads the Iranian Atomic Energy Organisation, said on Wednesday.
"Two or three months from then, the electricity generated by the plant will be connected to the grid," he added, pushing the timeframe to early 2011, not November as originally announced.
He has previously blamed the fuelling delays on "severe hot weather", and insisted on Wednesday that the plant was no longer being affected by the sophisticated Stuxnet computer virus that Iran's foreign ministry has described a "new game of soft warfare" by the country's enemies.
"We implemented measures to protect our computers last year, but during the past two months, these [cyber attack] activities increased dramatically," Mr Salehi said on state radio.
"Fortunately, we were able to neutralise the enemy's objective without involving the media. The fact these activities are continuing smoothly is evidence of this," he added.
Security experts say the worm may have been a state-sponsored attack on Iran's nuclear programme that originated in the United States or Israel.
Iran says it needs the Bushehr plant, under construction since the 1970s, to meet growing demand for electricity.
Officials say it will generate 1,000 megawatts, about 2.5% of the country's power usage when it is up and running.
Iran has been subject to four rounds of UN sanctions because of its uranium enrichment programme, which is separate from Bushehr.
The West fears Tehran wants to build a nuclear weapon, but Iran insists its plans are for peaceful energy production.