John McCain, the Arizona Republican, on Sunday signalled that he would not cave in to White House pressure to carve out an exception for the Central Intelligence Agency in legislation that would bar the US from torturing prisoners overseas.

Mr McCain, who was tortured as a prisoner of war in Vietnam, has for months led a Senate campaign to change US law to prohibit the “cruel, inhuman and degrading” treatment of prisoners. Those efforts have brought him into direct conflict with the White House, which has dispatched Dick Cheney, vice-president, to lobby lawmakers on the issue.