Serena Alexander-Benson left the country on a Eurotunnel train yesterday morning in Folkestone and was thought to have been with an older person.
The child was last seen by her father leaving her home in Wimbledon, south-west London around 7.50am on Friday.
Serena, who told him she was going to school, was wearing her green uniform.
The youngster, who has not disappeared before, did not arrive at her school and has been missing ever since.
Police are now trying to trace her.
She lives with her father in Wimbledon, and her mother lives in Poland.
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Detectives have appealed for information to help find her.
The Metropolitan Police said: “Anyone who may know Serena’s whereabouts or who may have seen her as she travelled through Folkestone – probably in the company of an older person – should call officers at Wandsworth CID on 020 3276 2588.”