A former governor of Pennsylvania, Ridge was picked by Bush to head the newly created department of homeland security after 9/11. He served as homeland security secretary from January 2003 to February 2005.

Set to be released Sep 1, Ridge's memoir will further fuel debate over the Bush presidency's high-handedness in dealing with national security issues.

But Bush's aides have denied these allegations.

'We went over backwards repeatedly and with great discipline to make sure politics did not influence any national security and homeland security decisions,' Bush's chief of staff Andy Card has been quoted as saying.

Former homeland security adviser Fran Townsend said: 'I am a little mystified... Never in my experience did I see any political influence exerted on the cabinet secretary.'