John McCain, the Republican frontrunner for the 2008 presidential election, has dramatically raised the stakes in a fight with the White House over interrogation techniques permitted for use at secret Central Intelligence Agency prisons by saying he is unwilling to back down on the issue even if it ruins his chance of becoming president.

The Arizona senator, who was tortured as a prisoner of war in Vietnam, strongly opposes Bush administration legislation that he says would redefine US obligations under the Geneva conventions. The White House played down the escalating dispute , which mirrors a similar battle Mr McCain won last year against the administration.