They are scheduled to release a statement on Wednesday attacking the president's foreign policy.
The 26 former officials - Diplomats and Military Commanders for Change - aired their views in interviews this week.
They say Mr Bush's policies have made the US more isolated and less safe, and damaged its standing in the world.
The BBC's Justin Webb in Washington says the group of many respected figures, includes some who worked for President Bush's father when he was in the White House.
Phyllis Oakley
Former state department official
The statement follows criticism last month by 53 former diplomats who accused the administration of undermining US credibility in the Arab world by its strong support for Israeli Prime Minister Ariel Sharon.
The UK government has also come under fire, with 52 former officials attacking Prime Minister Tony Blair's support for Washington over Iraq and the Israeli-Palestinian conflict.
'Dangerous posture'
Those signing the statement say they believe the Bush administration has made the international outlook more unstable and dangerous.
Phyllis Oakley, the former deputy state department spokesperson under President Reagan, told the BBC World Service's World Today programme that Mr Bush's Iraq policy had played a big part in their decision to publicise their concerns.
"But it goes beyond that to the whole thrust of his posture for the US and the world - to move away from the international structures that have been painstakingly built up over the years, away from our work with allies," she said.
Ms Oakley said it was a "dangerous posture" for the US to act as the "unilateral, sole superpower" that could impose its will on others.
"We cannot go it alone," she said.
In their criticisms of US policy on Iraq, the signatories say that all the assumptions made by the administration before the invasion have been proved wrong.
"It's a plea with the president to more urgently seek multilateral support for what we're doing in Iraq, to go back and forth in strengthening the alliances we've traditionally worked with," Admiral Crowe told the BBC.
'Failed policies'
The group is made up of both Democrats and Republicans.
Known critics of the administration were deliberately excluded.
Susan Taylor, Bronxville, NY
"It is clear that the statement calls for the defeat of the administration," Mr Harrop said.
But supporters of the administration said the former officials who signed the letter were simply trying to hide the inadequacy of their own policies.
Cliff May, president of the conservative Foundation for the Defense of Democracies, told the BBC: "Largely, they are people who were in senior official capacities before 9/11. They are people who are responsible for the policies prior to 9/11.
"Those policies I think, failed spectacularly on 9/11," he said.
Nevertheless, these are what some people might describe as 9/10 people, they want to continue with the failed policies and they don't want to change those policies despite their failures."
THE SIGNATORIES
Avis T Bohlen , Bush Jr's former assistant secretary of state for arms
control
Adm William J Crowe , chairman of the joint chiefs of staff
under Reagan and ambassador to UK under Clinton; has endorsed Kerry
Jeffrey S Davidow , Bush Jr's former ambassador to Mexico
William A DePree , ex-ambassador to Bangladesh
Donald B Easum
, ex-ambassador to Nigeria
Charles W Freeman , ex-ambassador to
Saudi Arabia
William C Harrop , Bush Sr's ambassador to Israel
Arthur A Hartman , ex-ambassador to Soviet Union and France
Gen Joseph P Hoar , commander in chief of US Central Command under
Bush Sr; supports John Kerry
H Allen Holmes , ex-special operations
chief
Robert V Keeley , ex-ambassador to Greece and Zimbabwe
Samuel W Lewis , ex-ambassador to Israel
Princeton N Lyman
, ex-ambassador to South Africa
Jack F Matlock , ambassador to
the USSR under Reagan and Bush Sr
Donald F McHenry , ex-ambassador to
the UN
Gen Merrill A McPeak , former Air Force chief of staff;
supports Kerry
George E Moose , ex-African affairs chief
David
D Newsom , former acting secretary of state
Phyllis E Oakley ,
ex-intelligence and research chief
James Daniel Phillips ,
ex-ambassador to Africa
John E Reinhardt , ex-ambassador to Nigeria
Gen William Y Smith , ex-deputy commander in chief, US European
Command
Ronald I Spiers , ex-senior UN official and ambassador to
Pakistan
Michael Sterner , ex-ambassador in Middle East
Adm
Stansfield Turner , CIA director under Carter; has endorsed Kerry
Alexander F Watson , ex-assistant secretary of state for
Inter-American affairs
