Izzard made the statement at a party rally in Brighton Thursday, The London Evening Standard reported.
"Eddie is a man of enormous substance, generosity and conviction. His decision to do this is a vote of confidence in Labor," the newspaper quoted Member of Parliament Tessa Jowell as saying.
The Standard said it is unclear why the 51-year-old entertainer has no plans to run for mayor in 2016.
The popular standup comedian has starred in the TV drama series "The Riches," as well as in the films "The Chronicles of Narnia: Prince Caspian," "Cars 2," "Valkyrie," "Ocean's 12" and "My Super Ex-Girlfriend."