In a close vote at Monday night's full council meeting, Croydon's ruling Labour group pushed through proposals that will replace the century-old committee system with a cabinet of nine and a leader.
Opposition Tories had hoped disgruntled Labour backbenchers would rebel against the plans.
They pleaded with their council colleagues on the opposite benches to "follow their consciences" and oppose the plans which they claimed would lead to an elected dictatorship.
But in the end Labour councillors towed the party line and the vote was carried by 36 to 32 with one abstention.
The result was greeted with jeers of "gutless" and "spineless" from the public gallery.
"Labour councillors are more interested in following the party whip than the long-term interests of democracy," said Tory Councillor Andrew Pelling.
"This will lead to government behind closed doors and power concentrated in the hands of the very few. It is very bad news for democracy."
The model chosen was one of three offered to Croydon residents in a recent consultation exercise.
Although the majority of respondents backed the council's choice of a cabinet and leader, the consultation was rubbished by Tories who said it was biased and unrepresentative.
In the next phase of change to start in May, the council will move to a transitional constitution as the new arrangements are brought on line.
As well as the creation of a cabinet and shadow cabinet there will be an expanded scrutiny role and cabinet consultative panels to consult on major issues.
The leader of the council, Councillor Hugh Malyan denied that the new arrangements would sideline backbenchers. Welcoming the year-long transitional constitution, he said: "We have produced a good transitional system. It combines the best of our existing system while freeing us up from the worse aspects. It will enhance the role of all councillors."
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