The prince, who is the Football Association’s president, met the players and manager Gareth Southgate at England’s St George’s Park national football centre in Burton-upon-Trent.
He presented shirts to the 26 players and delivered a speech, including some advice from his youngest child – six-year-old Prince Louis.
Prince William, an Aston Villa fan, told the players: “I was on the school run this morning with the children and I said, ‘What shall I say to the England team today?’
“The best bit of advice I got to ask you was to eat twice the amount you normally would eat.
“So I then had visions of all of you running around with massive great tummies and loads of stitches on the pitch, so I think maybe take my youngest advice with a pinch of salt.”
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Schoolchildren from the Midlands were invited to the event to mark England’s send-off.
Southgate’s squad will depart for Germany later on Monday before their first group game against Serbia on Sunday.
England, runners-up at Euro 2020, also face Denmark and Slovenia in Group C.
They lost 1-0 to Iceland in their final warm-up match on Friday.
The tournament starts on Friday, with hosts Germany against Scotland.