In an interview with the Evening Standard, the minister said he believed he had met Rashford, who forced the Government into a U-turn on free school meals provision during the pandemic, on Zoom.
But his advisers later told the newspaper that he had actually met rugby star Itoje.
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Hide AdRashford laughed off Mr Williamson's gaffe as he tweeted: "Accent could have been a giveaway."


Asked by the Standard whether he had met Rashford, the Cabinet minister told the paper: "We met over Zoom and he seemed incredibly engaged, compassionate and charming but then he had to shoot off. I didn't want to be the one that was holding him back from his training."
Shadow justice secretary David Lammy wrote on Twitter: "This is appalling.
"GavinWilliamson what was it about maroitoje that made you mistake him for MarcusRashford?
"You must be the most ignorant, clueless and incapable Education Secretary in the UK's history."
Last month, Labour leader Sir Keir Starmer said the Education Secretary should have been sacked "a long time ago" following heavy criticism over his performance in the role over the past year.
It came amid reports that Boris Johnson is considering replacing him at the next reshuffle.
But during Prime Minister's Questions on Wednesday, Mr Johnson defended Mr Williamson.
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Hide AdShadow schools minister Peter Kyle, Labour MP for Hove, asked: "With a net approval rating among Tory supporters of minus 53, can the Prime Minister get to his feet, put his hand on his heart, promise the country, this House and his own supporters that the Education Secretary is the right person for the job, and is up to the job?"
Mr Johnson responded: "I think the whole House will recognise that the Education Secretary has done a heroic job of dealing with a very difficult circumstance in which we have had to close schools during the pandemic.
The Education Secretary has said he made a "genuine mistake" after saying he had met footballer Marcus Rashford online when he had instead talked to rugby player Maro Itoje.