Supporters of Mr Mousavi may be louder rather than greater in number
The official result in Iran's disputed presidential election could plausibly reflect the will of the people, a group of international pollsters says.
An independent poll three weeks ago had Mahmoud Ahmadinejad ahead of his closest rival by a similar 2:1 ratio.
Runner-up Mir Hossein Mousavi has claimed the election result was fixed.
The research was conducted by US-based polling organisations Terror Free Tomorrow, the New America Foundation and KA Europe SPRL.
"We found that President Ahmadinejad was leading by a substantial margin," Ken Ballen from Terror Free Tomorrow told the BBC World Service.
The nationwide poll was conducted between 11 and 20 May and consisted of 1,001 random interviews covering all 30 provinces of Iran. It had a 3% margin of error.
Its results gave Mahmoud Ahmadinejad a 33.8% share of the vote, more than twice as much as Mr Mousavi with 13.6%, and with Mehdi Karroubi and Mohsen Rezai trailing on less than 2% and 1% respectively.
Respondents says none of the candidates in 7.6% of interviews, while 15.1% refused to answer and 27.4% said they didn't know.
"Whether or not this would have changed, or whether Mr Ahmadinejad would hold that lead which would have translated into a victory, that's where the unknown factors arise," Mr Ballen said.
According to official results Mr Ahmadinejad, the incumbent president, won 62.6% of votes cast. Mr Mousavi trailed with 33.8%.
"It's a plausible result, but the way the Iranian government handled it raises lots of questions," Mr Ballen told the BBC.
His polls predicted that no candidate would pass the 50% threshold for an automatic win, and a second round would take place between the two highest finishers.
In the 2005 presidential elections, the leader in the first round, Ali Akbar Hashemi Rafsanjani, lost to the runner-up, Mr Ahmadinejad, in the run-off.
Mr Ballen said the independent survey was a rarity in Iran, where polls are normally carried out by state agencies.
"It is not a society that allows independent polling or exit polling or election monitors or independent monitors so its very hard to ascertain whether or not the results actually reflect the will of the people," he said.
However, the large number of students now protesting against the results was also in keeping with the findings and did not necessarily reflect the will of the whole country, Mr Ballen said.
"The only groups we found in Iran that were supportive of Mousavi or [among whom]... he was competitive with Ahmadinejad were university students, university graduates and the highest-income Iranians."
Other groups, such as Azeris, to whom Mr Mousavi was considered likely to appeal because of his Azeri background, also showed stronger support for Mr Ahmadinejad ahead of Mr Mousavi.
Only 16% of Azeris said they intended to vote for Mr Mousavi, compared to 31% who said they would vote for Mr Ahmadinejad.
"We need to be cautious in drawing a conclusion," Mr Ballen said.
"We know it [the election] wasn't free and fair but to jump to the next conclusion that Mousavi would have won with a landslide, we don't have hard scientific evidence for that.
"But we do have evidence pointing in the other direction, that the result may have been valid."