British Foreign Secretary Mr Robin Cook today demanded an apology for a claim that the UK ambassador to Japan deliberately wrote a report highlighting unease about the Government's attitude to the euro so that it could be leaked.

Mr Cook said the claim by eurosceptic businessman Lord Hanson was an "outrageous slur" on Sir Stephen Gomersall.

Colleagues said Sir Stephen, the Foreign Office's foremost expert on Japan, would be "appalled" and "horrified" at the suggestion - and also at the leaking of his telegram in the first place.

Sir Stephen's report warned of unease among Japanese investors over Britain's stance on the euro. Lord Hanson said the leaking was "another example of spin". "We all know that Robin Cook is desperate to force this issue ahead."

Mr Cook said: "The suggestion that I instructed Sir Stephen Gomersall to doctor a report so that it could be leaked is an outrageous slur on a distinguished diplomat and on ministers."

Sir Stephen's telegram, marked "Restricted", was leaked to The Times and published on its front page yesterday. It warned that uncertainty over the Government's stance on the euro could "trigger a much stronger move towards disinvestment in the manufacturing sector".

It was just the latest leak in the row over the euro, attributed by the Tories to internal Cabinet squabbling over how quickly public opinion should be led towards Britain joining the single currency.