Ms Plame and her husband, Bush critic Joseph Wilson, filed a suit in July against Vice-President Dick Cheney; his former chief of staff Lewis “Scooter” Libby; and Karl Rove, George W Bush’s top political adviser, accusing them of intentionally revealing Ms Plame’s identity more than three years ago to discredit Mr Wilson.
But a Newsweek magazine report written by Michael Isikoff, the co-author of the book, Hubris, said that Mr Armitage was “a member of the administration’s small moderate wing” who let the information slip to columnist Robert Novak because he is “a well-known gossip who loves to dish and receive juicy tidbits about Washington characters”.
The BBC reports that since the Newsweek article first appeared, a number of other news organisations have said they have independently confirmed the Mr Armitage’s role.
While no-one has been charged with leaking the information, the case forced Mr Libby to resign as Mr Cheney’s chief of staff when he was charged with obstructing a special prosecutor investigating the case.
However one of Ms Plame’s lawyers told the BBC the book’s new information “has no impact on the case”.
”This case is about top-level White House officials who conspired against Valerie Wilson,” Melanie Sloan told the BBC, referring to Ms Plame by her married name.
"If for some reason we find Mr Armitage was more involved, then we can add him as a defendant,” she added. “It doesn't change the case against Cheney, Libby and Rove."
Mr Armitage has made no public comment on the claims.