Do Muoi, an octogenarian who was Phieu's predecessor as party secretary general, said it was time for a younger generation to take over and said he himself would also step down as a party adviser.

Asked why Phieu would not continue in his post after a five-yearly party congress due to end on Sunday, Muoi told reporters: "Because he is getting old."

"He is in the same tenure as me and at the end of the term we finish the job to pave the way for rejuvenation."

A senior government official on Thursday confirmed reports that reform minded National Assembly chairman Nong Duc Manh, reputedly a son of revolutionary leader Ho Chi Minh, would take over from the unpopular conservative Phieu.

"His contribution is great but he also has made mistakes, mistakes in his work," Muoi said of Phieu, who only took his job in 1997. He did not elaborate.

Many party sources have referred to Phieu, a former political commissar, as ineffectual and out of touch with a reformist trend. Highly critical reports circulating in recent weeks showed the strength of pressure to get rid of him.

They have included allegations he used the intelligence apparatus to monitor his colleagues in the party's elite politburo and that he gave too much away to China in border agreements.

Phieu told Reuters in a rare interview on Thursday he was willing to make way for a younger man if the congress so decided and the conditions were right.

Muoi and President Tran Duc Luong refused to be drawn on who would succeed Phieu, saying it was up to the congress to decide, but neither ruled out Manh.

A formal vote on the leadership will be taken on Saturday with an announcement of the result to be made on Sunday.

However, the government official said the decisive vote on the new leadership had already been taken at an meeting of the party's new 150-member central committee on Tuesday. Party terms are for five years.

Luong, expected to stay in his post for the time being along with reformist Prime Minister Phan Van Khai, told reporters "the entire party and people" hoped Vietnam's reform process would be pushed forward as a result of the congress.

In saying he would retire as a party adviser, Muoi suggested all three advisory posts would be discontinued.

Diplomats said this would be a very positive development.