Nearly six months after ballots were cast, the secret vote that awarded the hotly contested 2012 Olympics to London rather than New York is the subject of a new debate.

Olympic officials scrambled yesterday to shoot down reports that a delegate of Greece miscast his vote in the July 6 ballot that brought victory to London, disappointment to many New Yorkers, and shock to Paris.

London defeated Paris in the fourth and final round of electronic voting, but the two-city race emerged only after Madrid was eliminated in the previous round. None of the five competing cities challenged the results at the time, but on Friday the British Broadcasting Corp. reported that a member of the International Olympic Committee had intended to vote for Madrid in the third round, but had mistakenly voted for Paris. Later reports named the IOC member as Lambis Nikolaou of Greece.

The error could have had enormous consequences, since Madrid only trailed Paris by two votes during the third round, and a switch of just one member would have forced a tiebreak between the two cities.

In a delayed response to the reports, Mr. Nikolaou yesterday denied that he had voted at all, and the IOC declared the matter irrelevant.

Mr. Nikolaou initially remained silent after the BBC quoted an Israeli IOC member, Alex Gilady, as saying that an IOC member had miscast a vote. But Mr. Nikolaou responded yesterday in an e-mail statement to news outlets in Athens. “I declare that I didn’t vote in the third round, as I stated at the time of the voting,” Mr. Nikolaou said, according to Bloomberg News.

The IOC corroborated Mr. Nikolaou’s account. In a statement posted on the IOC Web site, the committee said 104 ballots were distributed in the third round, and 103 were cast. “No abstentions were recorded; no votes were voided,” the statement said. The IOC added that it would have been mathematically impossible for the 104th vote to change the outcome, since the margin between Paris and Madrid was two votes, not one.

London went on to defeat Paris by four votes in the final ballot. New York and Moscow had been eliminated in the first two rounds. Yet the dispute reflects the high-stakes nature of the years-long Olympic bid process in which cities spend millions of dollars to win the backing of the IOC.

In responding to the reports of a voter error, the IOC is seeking to head off controversy after past selections were marred by allegations of bribery and corruption.

An American IOC member, Robert Ctvrtlik, expressed doubt that a delegate could make a mistake casting an electronic ballot, noting that “the primary responsibility we as IOC members have is voting.” The voting, he said, is conducted on a system similar to the one used at the United Nations. Each delegate has an electronic box with buttons corresponding to each choice, and instructions are given simultaneously in several languages.

“Anyone with a first-grade education could operate the box,” Mr. Ctvrtlik said. He described the voting process as “honest” and “extremely thorough.”

“They actually go through the instructions many more times than any human being needs to hear,” he said.

At its session in Singapore in July, IOC members voted on dozens of issues ranging from executive board member positions to procedures for the games. The four rounds of voting on the 2012 host city took about an hour, Mr. Ctvrtlik said.

The voting dispute offers little solace for New York, which put forth a bid championed by Mayor Bloomberg as an opportunity for the city to play host to the world and generate an economic windfall in the process. New York lost in the second round in Singapore, capturing only 16 votes.

A longtime political observer and former city parks commissioner, Henry Stern, likened the situation to the firestorm that erupted after the 2000 presidential election in Florida, where thousands of voters misread the “butterfly” ballot in Palm Beach County. “It’s the principle of fair elections,” Mr. Stern said. “It’s the principle that once something is decided, it’s decided.”

The expectations for an Olympic vote are even higher, he said. “The guy voted by mistake? Give me a break,” Mr. Stern said. “If you’re an Olympic voter, you’re presumed to know how to read and write, and put your check in the right box.”

Two leaders of New York’s 2012 bid, Daniel Doctoroff and Jay Kriegel, declined to comment.