More than 1.5m more people would be eligible to vote if the franchise was expanded to include 16 and 17-year-olds - enough to swing dozens of seats, Labour has claimed.
The Opposition has pledged to lower the voting age if it wins the General Election in time for a series of elections due in May 2016, including for London mayor and the devolved assemblies.
Shadow justice secretary Sadiq Khan said the high turn out of 16 and 17-year-olds in last year's Scottish independence referendum demonstrated youngsters want to be involved.
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Mr Khan said: "Labour wants the voices of our young people to be heard. We will give 1.5 million 16 and 17-year-olds the right to vote in elections from May 2016 onwards, the biggest expansion of the franchise in nearly half a century.
"This new data belies the myth that young people can't make a difference. If the Tories had supported Labour's calls to lower the voting age, then the number of 16 and 17-year-olds would have exceeded the sitting MP's majority in seats across the country on General Election day.
"The potential for the youth vote swinging the result is enormous. This will make all parties sit up and respond to the needs of younger voters."
Labour highlighted scores of seats currently held by the Conservatives and Liberal Democrats which have majorities smaller than the estimated number of 16 and 17-year-olds.
They include tight marginals such as Hendon, won by Tory Matthew Offord by 106 votes in 2010, which had in mid 2013 about 3,266 16 and 17-year-olds.
In London, about 187,000 more people would be eligible to vote after the change - bigger than both of Boris Johnson's majorities and that won by Ken Livingstone for his second term at City Hall.Suggest a correction