Havering's cabinet has agreed to commit the council to reach net zero emissions by 2040 and an action plan to tackle the climate crisis.
The document sets out actions that different departments will take, with a council report described by corporate projects manager Nick Kingham as one of the most important ever presented.
The council's bid to reach carbon neutral within the next 20 years is a decade ahead of the government's targets.
But opposition leader Ray Morgon said the report contained "a lot of the what, but not the how and when".
He urged the authority to prioritise the order of their actions, adding that he would be surprised if the target was met.
Among the measures proposed are retrofitting buildings to improve energy performance, improving the council's resilience and response to climate change-related events such as flooding and reviewing the capital spending strategy so it supports net zero emissions.
Government figures from 2019 show the transport sector accounts for almost half of carbon dioxide emissions in Havering.
Proposals to reduce transport emissions include reviewing the borough's fleet of vehicles and ensuring none are diesel-powered, developing a cycling and walking strategy and planting at least 250 trees a year on council land.
The cost of the proposals is "significant but impossible to estimate", Mr Kingham told November 10 cabinet meeting.
Cllr Morgon said he was concerned at how the council would achieve the aims considering its multi-million pound budget deficit.
He added: "There are significant costs which have not been identified and where the funding is coming from.
"We need to take residents on a journey but there will be a cost to address a lot of these issues and when the cost of those is explained to residents, they may not be so keen."
Labour group leader Keith Darvill said he welcomed the report but felt it had taken too long to materialise.
Other recommendations agreed by cabinet include a commitment to publish emissions data on a yearly basis and the production of a six-monthly report on the action plan's progress.
The discussion came amid the backdrop of the global COP26 conference on climate change in Glasgow.
Council leader Damian White said: "It is something we cannot ever allow to be put on a shelf and allow dust to cover.
"This report is meant to be part of our DNA and what we have to do as an organisation to tackle climate change."
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