The confirmation came from the Thomas & Friends UK page in response to a fan's query about the film's cinematic arrival.
While times are yet to appear, the film's launch foresees a three day weekend. King's first indication of an appearance in August came to light at the tailing end of March.
King of the Railway: The Movie premieres in Vue cinemas nationwide across the UK on the weekend of August 23rd 2013 with its DVD to follow in September.
Thanks to lg77612 for the alert.