The yen is appreciating “too fast” and speculators need to “be careful”, Yasuo Fukuda, Japan’s prime minister, warned in an interview with the Financial Times on Monday.

As the yen moved to an 18-month high of Y109.13 to the US dollar on Monday night, Mr Fukuda said: “In the short term, yen appreciation would certainly be a problem. Any kind of sudden change in exchange rates would not be desirable.”