Problems with electronic voting machines during Florida's presidential election may have caused 260,000 extra votes to be mistakenly attributed to George W Bush, a Berkeley professor has claimed.

Professor Michael Hout of the University of California, Berkeley has published a study alleging that "compared to counties with paper ballots, counties with electronic voting machines were significantly more likely to show increases in support for President Bush between 2000 and 2004".

He stated that the possibility of other factors accounting for the discrepancy, like a change in voting patterns, was less than one in a thousand.

"The disparity in favour of the incumbent cannot be explained by other factors," he said.

"For the sake of all future elections someone must investigate and explain these results. The results are like a smoke alarm and we want Florida officials to investigate the bleeping."

Professor Hout said that the counties with the largest discrepancies between predicted and actual results were the traditional Democrat strongholds of Miami-Dade, Broward and Palm Beach.

In Broward County, for example, the study found that Bush appears to have received approximately 72,000 excess votes.

Professor Hout explained that he was sceptical of the original idea, but that the findings had changed his mind. The study used a widely accepted practice of multiple-regression analysis, which seeks to eliminate errors to provide a valid statistical base.

The results in Ohio were also examined, but no discrepancies were found. In both Ohio and Florida the actual results cannot be counted as no paper receipts are issued.

Professor Hout is an expert on statistical methods at the Berkeley Survey Research Center and a member of the National Academy of Sciences.

The study, entitled The Effect of Electronic Voting Machines on Change in Support for Bush in the 2004 Florida Elections, is available here.