By Paul Reynolds
World Affairs correspondent, BBC News website
The British accusation that Iranian elements are behind lethal attacks on British troops in southern Iraq came out after months of frustration.
The British ambassador to Baghdad, William Patey, has time and again complained to his Iranian counterpart that there is a traceable link between bombs that have killed eight British soldiers and devices used by Hezbollah in southern Lebanon, which is backed by Iran.
British soldiers have been forced onto their guard in recent weeks
The Iranians have always denied any such link.
Up until now, the British approach has been the classic one of hints and suggestions.
The official who was the source of the accusation in a briefing for correspondents had previously been much more circumspect.
This time he was quite open and direct. And having just come from a meeting with the prime minister, he was not speaking by accident.
He spoke of the "explosively shaped projectiles" which were now such a threat to British armoured vehicles.
At one point he even said it was possible the Iranians had "woken up one morning and decided that they weren't gong to be pushed around".
The "Iranians" he defined as the Revolutionary Guards. Asked if the Iranian government itself might be involved, he smiled and said he would not comment.
He then named the alleged leader of a dissident Shia group in Basra as being behind the attacks.
"Fartusi", he murmured, referring to Ahmed al-Fartusi, whom the British arrested recently.
Iran's relations with Britain have soured over the nuclear issue
So what has changed from before?
What has changed, I think, is that relations between Iran and Britain have become so low that the British felt they had nothing to lose.
And relations are so low because of the failure of the European effort to get Iran to give up its efforts to master the nuclear fuel technology.
Indeed, Britain feels it has something to gain by making this charge known and hoping that Iran will itself take action in order that wider relations might be improved.
So while a few months ago this official was cautious, this time he was accusatory.
There was no effort by the Foreign Office news department representative present to get the accusation stricken from the record.
The only rule was that the official's name could not be used, a common device in the briefing of journalists.
Indeed as he left, the official joked that he would be in trouble if he had not been "on background" as the jargon calls it. So he knew what he was doing.
One cannot of course judge the accuracy of the accusation.
And there are questions to be asked about how Britain might have made the connection with Hezbollah.
Britain has no experience of Lebanon. So how would it know that bombs used in southern Iraq could be connected to bombs used by Hezbollah?
Fatal attacks have risen noticeably in the past few months
The answer has to involve the Israelis, with perhaps the Americans acting as intermediaries.
Who other than Israelis and Americans would know how Hezbollah makes a bomb? The Lebanese are unlikely to tell.
This in turn opens up all kinds of interesting issues. For one thing, the intelligence relationship between Britain and Israel, with or without the US, must be quite close these days.
This has not always been the case. But mutual dangers often bring mutual support.
As for Iran, no doubt the rhetoric will be raised.
In the old days, such accusations would have meant war. In 1739 a British naval Captain, Robert Jenkins, complained that a Spanish commander had cut off his ear.
The severed object was shown to the House of Commons and war was subsequently, if reluctantly, declared.
That will not happen this time.
But a hardball round of diplomacy is in play.