By a 90 to nine vote, the Senate tacked the demand on to to a $440bn (£250bn) Pentagon spending measure, despite a personal lobbying effort by Vice- President Dick Cheney, and a threat by the White House to veto the bill if it still contained the restrictions. The administration maintains that such curbs would harm its ability to fight the "war on terror".
But astonishingly, 46 of the 55 Senate Republicans, led by John McCain, held firm. The Arizona Senator, who spent seven years as a prisoner-of-war in Vietnam, said that many military officers had begged Congress to provide clear guidelines, in contrast to the ambiguous instructions sent out by the Pentagon.
The revolt suggests that Republicans - even strong supporters of the 2003 invasion such as Mr McCain - are now ready as never before to challenge Mr Bush's handling of the Iraq war, now approved by barely 30 per cent of Americans, according to recent polls.
The provision would set uniform standards for interrogation, banning "cruel, inhuman or degrading" treatment of prisoners. It was also backed by General Colin Powell, the former secretary of state, who said it would help deal with "the terrible public diplomacy crisis created by Abu Ghraib". To become law, the Senate military spending bill must be reconciled with a parallel measure in the House, where White House support is stronger. But after almost five years in office, Mr Bush has yet to veto a single spending bill sent to him by Congress.