LONDON (Reuters) - British Prime Minister David Cameron said on Thursday that his main priority was to ensure a safe and secure Olympic Games but that police and security services were leaving nothing to chance just a day before the opening ceremony.

"This is the biggest security operation in our peacetime history, bar none and we are leaving nothing to chance," Cameron told reporters at the Olympic Park.

When asked by a reporter what his biggest worry was, he said: "Obviously the biggest concern has always got to be a safe and secure Games. That matters more than anything else."

Cameron said the gaffe with the South Korean flag being displayed before North Korea's women's soccer match against Colombia was an honest mistake.

He said he was sure every effort would be made to prevent such a mistake happening again.

(Reporting by Stephen Addison and Guy Faulconbridge)