Gambari was supposed to have addressed the East Asian summit on Wednesday about the progress he has made in recent meetings with Myanmar's ruling generals.
The invitation was extended by Lee Hsien Loong, Singapore's prime minister and current chairman of Asean, who said later: "It is a very difficult matter. It is a difficult problem for Myanmar."
He said Thein Sein, Myanmar's prime minister, had insisted during a dinner meeting that Gambari should only report to the UN, and not Asean or the East Asian summit.
"Until the generals' military hardware is crumbled, they won't listen to anyone"
Oomlwin, Yangon, Myanmar
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Ban Ki-moon, the UN secretary-general, said he had not been officially informed of the cancellation of Gambari's speech.
"It would have been desirable and better for Gambari to report and brief the East Asia summit leaders on the situation in Myanmar,'' he said, adding that he needed to discuss the matter with Lee.
"My commitment and United Nations' commitment to see and help [the] democratisation process of Myanmar is unchanged [and] will continue."
Lee said that following Myanmar's objections, Asean leaders agreed to "respect Myanmar's wishes and make way for Myanmar to deal directly with the UN and the international community on its own".
He said, however, that most Asean leaders felt that Myanmar "could not go back or stay put" and that the UN plays a pivotal role in pushing the process of national reconciliation in Myanmar forward.
Diplomats said the objection to Gambari's address was not only from Myanmar but from other countries as well, notably Malaysia and Indonesia, who felt that it would amount to interference.
The 10-nation Asean grouping, which includes Myanmar, has a policy of non-interference in the domestic affairs of member states and has rejected calls for Myanmar's suspension.
The East Asia summit brings together Asean and six others: China, India, Japan, South Korea, Australia and New Zealand.