US presidential hopeful Mitt Romney made some undiplomatic criticism of London's preparations for the Olympic Games on Wednesday, expressing concern about Britain's readiness to host the event.

"It's hard to know just how well it will turn out," Romney, who is running to unseat President Barack Obama in November's election, told NBC News from London, where he will attend the opening ceremony of the games on Friday.

Outspoken ... Mitt Romney. Credit:AP

"There are a few things that were disconcerting," he said.

"The stories about the private security firm not having enough people, the supposed strike of the immigration and customs officials - that obviously is not something which is encouraging," he warned.