David Hackworth, a retired US army colonel turned writer, reported that Mr Rumsfeld had used a mechanical signature writer to sign his name on letters of condolence to relatives of soldiers killed in Iraq and Afghanistan.

Although the charge was initially denied by the Pentagon, Mr Rumsfeld later issued a statement acknowledging the practice and promising to halt it.

“While I have not individually signed each one, in the interest of ensuring expeditious contact with grieving family members, I have directed that in the future I sign each letter,” Mr Rumsfeld said in the statement.

But the revelations about Mr Rumsfeld cap an uncomfortable fortnight for the defence secretary.

One of the few members of the Bush cabinet to avoid being reshuffled after the election, Mr Rumsfeld has come under attack from within the Republican Party for his handling of the war in Iraq.

Public attention focused on the defence secretary when he faced hostile questions in a town hall-style meeting with Iraq-bound soldiers in Kuwait. He bluntly answered their complaints about a lack of equipment by saying that governments have to go to war with the army they have, “not the army you might want or wish to have at a later time”.

Critics have focused on his sometimes arrogant style, his handling of the Abu Ghraib prison abuse scandal, and his enthusiasm for a scaled-down, hi-tech military.

Mr Rumsfeld also came under attack from the veteran Republican and former senate majority leader Trent Lott last week, who declared that he was “not a fan” of the secretary of defence.

And William Kristol, the editor of the conservative Weekly Standard and one of the biggest supporters and architects of the neo-conservative foreign policy followed by the Bush White House, questioned whether Mr Rumsfeld was the right man for the job.

Most observers concur that it is very unlikely that President George W Bush will remove Mr Rumsfeld before the scheduled January elections in Iraq.

But with more than 1,300 US soldiers killed in Iraq, the revelation that Mr Rumsfeld has not personally signed all the condolence letters will heighten the impression that the administration has been callous in its handling of the war.