Labour must learn the lessons of its by-election defeat in Uxbridge, Sir Keir Starmer has said.
The Labour leader had blamed the loss on London Mayor Sadiq Khan's plans to expand the Ultra Low Emission Zone (Ulez) - a tax on polluting vehicles.
Conservative Steve Tuckwell won the seat after campaigning against the tax.
Addressing Labour's national forum, Sir Keir said there was "something very wrong" when a Labour policy was on "each and every Tory leaflet".
He said that while a by-election win in Selby and Ainsty, in North Yorkshire, should give Labour "every reason to be confident", the loss in Uxbridge showed there is "still a long way to go".
"That result in Uxbridge demonstrates there is never any reason to be complacent and never a reason to rest on our laurels," Sir Keir told the forum in Nottingham.
"We've got to face up to that and to learn the lesson," he said.
Labour's Keir Mather, 25, won the contest in North Yorkshire on Thursday, overturning a 20,137 majority to become the youngest sitting MP.
But the Conservatives clung onto ex-PM Boris Johnson's former Uxbridge seat, sparking debates about both parties' green policies.
Sir Keir told the BBC the Ulez plan had cost Labour victory - but Mr Khan has defended the measure as the "right one".
Mr Tuckwell, the winning candidate, said the "damaging and costly Ulez policy" had lost Labour the seat.
Some on the right of the Conservative party say that pulling back from some green policies would prove popular with voters, at a time when families are feeling cost-of-living pressures.
Tory MP Craig Mackinlay, chairman of the Net Zero Scrutiny Group, has suggested delaying the ban on new diesel and petrol cars, pushing it back "at least" five years to 2035.
Downing Street sources say there are no plans to change climate targets - but that Prime Minister Rishi Sunak will try to set his party apart from Labour in the coming months.
As the major parties digest the by-election results, ex-climate minister Lord Ian Duncan, a Conservative, warned that if Sir Keir and Rishi Sunak do not put politics aside and agree a common approach to climate change, people will face "serious challenges".
Lord Duncan, who was the parliamentary under secretary for climate change from July 2019 to February 2020, said a "bipartisan approach" was needed from both parties to "get behind" common climate policies.
Politicians might win votes in the short-term by distancing themselves from strong climate policies - "but medium term, I'm not even talking long term anymore, there will be serious challenges and changes to our climate that will affect people in their everyday lives", he said.
However, speaking on BBC Radio 4's Today programme, Lord Duncan said the challenge was ensuring climate policy did not penalise people "beyond their ability to pay".
Referring to greener technology such as new gas boilers, he said: "We've got to make sure it's a transition and it works for everybody."
No one should be left behind or be impoverished by these policies, "otherwise it will be a problem for democracy", he said.