LONDON, England (CNN) -- A survey of Iran's election results raises "serious questions" about the victory that President Mahmoud Ahmadinejad is said to have won and uncovers irregularities in the official results, a British think tank said Sunday.

Iranian expatriates protest the June 12 presidential election results on Sunday in Berlin, Germany. more photos »

Official statistics obtained from Iran's Ministry of the Interior show the votes cast exceeded the number of eligible voters in two provinces, said Chatham House, a London-based institute that analyzes international affairs.

Claims that Ahmadinejad, the conservative incumbent, swept the board in rural provinces also flies in the face of previous results, said Chatham House, which conducted the survey with the University of St. Andrews in Scotland.

Release of the survey results comes on the heels of violent demonstrations that followed Iran's disputed June 12 presidential election.

Anti-government demonstrators have protested the results in street rallies and marches, defying warnings from Iran's supreme leader, Ayatollah Ali Khamenei, not to engage in such action.

Iran's foreign minister Sunday disputed allegations of ballot irregularities. Watch amateur video of the past week's violence »

For the survey published Sunday, researchers worked from the province-by-province breakdowns of the 2009 and 2005 results, released by the Iranian Ministry of Interior, and from the 2006 census as published by the official Statistical Center of Iran, Chatham House said.

The survey made four main observations:

In two conservative provinces, Mazandaran and Yazd, a turnout of more than 100 percent was recorded.

At a provincial level, there is no correlation between the increased turnout and the swing to Ahmadinejad. This challenges the notion that his announced victory was due to the massive participation of a previously silent conservative majority.

In a third of all provinces, the official results would require that Ahmadinejad had received not only all former conservative voters, all former centrist voters and all new voters but also up to 44 percent of former reformist voters -- despite a decade of conflict between these two groups.

In the 2005 election, as in the elections of 2001 and 1997, conservative candidates -- and Ahmadinejad in particular -- were markedly unpopular in rural areas. That makes it "highly implausible" that the countryside swung substantially toward Ahmadinejad.

"The analysis shows that the scale of the swing to Ahmadinejad would have had to have been extraordinary to achieve the stated result," said professor Ali Ansari, a co-author of the survey who is director of the Institute for Iranian Studies at St. Andrews.

Data from the June 12 election suggests a sudden shift in political support toward Ahmadinejad in rural areas, which had not previously supported him or any other conservative, the survey said.

At the same time, the official data suggests the vote for challenger Mehdi Karrubi -- who was extremely popular in the rural, ethnic minority areas in 2005 -- has collapsed entirely, even in his home province of Lorestan, the survey said.

In that province, his vote went from 55.5 percent in 2005 to 4.6 percent in the most recent vote, the survey found. At the same time, Ahmadinejad won 50.9 percent of the vote in this election, including the votes of nearly half (47.5 percent) of those who voted for reformist candidates in 2005, the survey found.

Such an outcome is "highly implausible," the survey said.

The Iranian government has said Ahmadinejad won the election by a huge margin. Ahmadinejad's main challenger, Mir Hossein Moussavi, and his supporters have contested the results.

"In a country where allegations of 'tombstone voting' -- the practice of using the identity documents of the deceased to cast additional ballots -- are both long-standing and widespread, this result is troubling but perhaps not unexpected," the Chatham House survey said.

"This problem did not start with Ahmadinejad," the report added. "According to official statistics gathered by the International Institute for Democracy and Electoral Assistance in Stockholm, there were 12.9 percent more registered voters at the time of Mohammed Khatami's 2001 victory than there were citizens of voting age."

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