Sources in Cairo said the two countries fear that they are first on the Washington wishlist for political, social and economic reform in the region and declined an invitation to attend the summit in Georgia on the US southeast coast.
Tunisia, which holds the rotating presidency of the Arab League, has followed suit, but the leaders of Algeria, Bahrain, Jordan and Yemen have accepted Bush's invitation.
The initiative targets a region known for largely autocratic rule that stretches from Pakistan to North Africa, and includes Iran, Iraq, Lebanon and Syria.
Egyptian President Husni Mubarak and Saudi Crown Prince Abd Allah bin Abd al-Aziz, the de facto ruler of the kingdom, believe the plan risks turning the Arab world into an amalgam of widely divergent interests and first signalled their resistance to it in February.
Cairo and Riyadh have repeatedly signalled they would reject any attempt to impose a "foreign order" on the region, while pointing out they were not opposed to democratic reforms implemented by Arab states on their own initiative.
Washington, with its Broader Middle East and North Africa Initiative, sees reform in the Middle East as vital to easing the political frustration blamed for promoting anti-US terrorism.
Too little focus
With a spate of attacks by insurgents in the past year, Saudi Arabia feels it has already paid a high price for the turmoil in occupied Iraq, while Cairo is taking security steps to avoid a similar fallout.
Ahmad Mahir says NATO has no
role in Middle East political reform
The two countries have also criticised the US plan for putting too little focus on reviving the Palestinian, Israeli peace process.
Egyptian Foreign Minister Ahmad Mahir has said that NATO has no role to play in driving political reform in the Middle East, as envisioned in Bush's plan.
The Arab League, at its summit in Tunis in May, voiced strong reservations about the plan but refused to consider Egypt's counter-proposal for a top-level Arab body to coordinate and monitor reforms in the region.
Egypt, which claims it has long been implementing incisive reforms, has tried various ploys to take the initiative out of American hands, including urging its intellectuals to speak up on the subject.
Mubarak in March hosted a summit in the port city of Alexandria of Arab non-governmental organisations, bringing together 165 political figures from 18 countries.
Jordan's King Abd Allah has
accepted Bush's invitation
The organiser of the unprecedented gathering, Ismail Siraj al-Din, head of the Alexandria Library, told delegates that "reform in the Arab world has become a vital matter because in a rapidly changing world there is no place for immobility".
But Arab states appear adamant they will not accept an alien package of ready-made political and economic reforms.
The Tunis summit of Arab leaders closed with the adoption of a 13-point programme urging them to pursue reform at their own pace and in their own manner.
The sentiment was reiterated on Monday by Jordan's King Abd Allah II, said: "Any reform process should emerge from within - ownership of the process of reform is vital for its success - and initiatives seen as imposed from the outside will only hurt the efforts of genuine reformers in our region."