Election bosses are investigating allegations that polling station staff in Lambeth have spoilt ballot papers by writing on them.
London Elects said it was investigating what it described as an "isolated incident" in Lambeth where polling clerks had written what is thought to be the voting card numbers on 10 people's ballot papers.
A spokesman said any mark other than the voter's cross on the ballot paper made it invalid.
Around 5.5million registered voters are heading to the polls today to choose the city's mayor and 25 members of the London Assembly.
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