The Titanic actress, who appeared with Branagh in a 1996 movie adaptation of the Shakespearean tragedy, had been due to star in his new drama called The Guernsey Literary and Potato Peel Pie Society.
However, Winslet has now left the project and Branagh, who was due to direct, has been replaced by My Week With Marilyn moviemaker Simon Curtis.
Producer Paula Mazur has revealed a new leading lady has been chosen, but won't be announced until the deal is finalised.
She adds, "Making a movie is always a saga, and this one is no exception."
The movie is based on the novel of the same name by Mary Ann Shaffer and Annie Barrows and focuses on residents of the Channel Island of Guernsey, a British territory off the coast of France, who outwitted German occupiers during the Second World War.