In his letter to the Liaison Select Committee on Saturday — highly unusual for its timing — Gordon Brown insisted that the case for intervention in Afghanistan in 2009 was just the same as in 2001.

It was, he wrote, to take on the terrorist threat at source and prevent attacks in Britain and elsewhere. Yesterday he went further and said the mission represented a “patriotic duty”.

But one reason why Mr Brown had to use a letter to a select committee to get his message across was because he sensed, rightly, that public and political support is wavering.

Nick Clegg, the Lib Dem leader, has spoken of lives being “thrown away” by politicians. David Cameron and senior Conservatives have been raising worries about equipment, and public surveys suggest people do not know why the war is being fought.

The political consensus that backed the 2001 war after the terrorist attacks on September 11, 2001, has broken and Mr Brown faces a desperate and probably impossible task to put it back together again.

While there has apparently been no criticism in Cabinet of the mission, some have argued that it needs to be explained far better. From the moment he heard of the surge in deaths on Friday, Mr Brown has been speaking of the “chain of terror” stretching from the Afghan plains to the streets of Britain.

It is an argument he will put with increasing force in the days ahead. It will be difficult. This time there are strains between the military and politicians not evident in 2001.

There is scarcely concealed fury in government at the regular criticisms of General Lord Guthrie of Craigiebank, while No 10 also had to distance itself from criticism by General Sir Richard Dannatt, Chief of the General Staff.

It has been a hard summer and it is not over, Mr Brown said sombrely on Friday. He needs the public to start understanding this war.

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