Sadiq Khan has told the Labour party conference "we need to learn from where we got things wrong".
The shadow transport secretary claimed the previous government "made great strides" on bus services and carbon emissions but could have done more.
"We have set out some of the most detailed plans in the world, not just on how to cut emissions but also how to support greener motoring, create jobs and ensure that it is in the UK that we manufacture the clean cars of the future," he said.
"But we didn't always get the answers right and we need to recognise that and move on.
"Part of moving on means working with this government when they make good decisions, where we agree with them we should support them.
"But where they put our transport system at risk we should say so as well."
Khan told conference that the coalition government may put at risk investment in new trains, high speed rail and support for bus companies to provide services in deprived areas.
"Of course, we will support responsible cuts when times are hard, but right wing ideological cuts are wrong, unacceptable and we will expose them.
"Under David Cameron, much of what we gained is at risk.
"Passengers will not pay more for less."
Khan, who ran the Ed Miliband campaign and is expected to be appointed to a prominent shadow cabinet job, told conference that Labour must be prepared to make radical change as a party.
"I believe that we have that vision in us and we've shown the world this week that we're coming back, bolder than ever," he said.


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