Social work veteran Ho Hei-wah, who backed Leung Chun-ying to be chief executive, says the city's leader "betrayed" his support by breaking his election promises in his "extremely disappointing" policy address.
The director of the Society for Community Organisation (Soco) said he did not regret supporting Leung during his campaign - though neither did he have a "crystal ball".
Ho said Leung's maiden policy address last week contained minimal measures to ease poverty and level the playing field for people who were struggling to survive.
"Now it's time to tell [Leung] loudly that he's done a very bad job and hope that he'll listen," Ho said.
He said Leung had promised a redistribution of resources to the poor and new ways to ease social problems during campaigning, but that the address contained nothing new.
"It was a big disappointment. I feel that he betrayed [my support]," said Ho.
Leung had previously explained there were no immediate "sweeteners" to the community but said Financial Secretary John Tsang Chun-wah would be working hard on the February budget to announce more detailed measures to help the needy.
Ho said a lack of new ideas showed Leung's cabinet was weak, unable to propose good policies and work cohesively. This could be an Achilles' heel for Leung's government in future.
"[His policy address] still focused on economic development as a way to solve poverty, which we know from the past decade doesn't work," said Ho.
"The approach is no different from [the previous government's]," he said.
The Soco chief said that not only were there few new plans, Leung also failed to improve on current policies for the poor.
These included transport subsidies, medical subsidies for the elderly and schoolbook fees for poor children, which had already been discussed repeatedly with a general consensus reached in society.
Ho said he had known Leung for more than 20 years, and thought him a good leader and a good person.
They had worked closely in 1996 when both were part of the government's working group on the long-term housing strategy.
He also said "I don't have a crystal ball, I couldn't tell what would happen in the future", in reference to the disclosure of illegal structures at Leung's home and his policy address.
During the election campaign, Ho was criticised for his support of Leung, and his organisation was accused of "turning pro-government" with rumours that they had received government money.
Ho vehemently denied such accusations and said many, like him, decided to support Leung because he seemed willing to break the mould and change the approach to welfare.