Sir Keir Starmer has said Labor must learn the lessons of the Uxbridge by-election defeat.
The Labor leader blamed London Mayor Sadiq Khan’s plan to expand the Ultra Low Emissions Zone (Ulez) – a tax on polluting vehicles.
Conservative Steve Tuckwell won the seat after campaigning against the tax.
Addressing Labor’s national forum, Sir Keir said there was “something very wrong” with Labor policy being put on “every Conservative Party leaflet”.
He said that while winning the by-elections in Selby and Ainsty, in North Yorkshire, would give Labor “every reason to be confident”, the loss of Boris Johnson’s former seat in Uxbridge showed “there is still a long way to go”.
“That result at Uxbridge proves that there is never any reason to be complacent and there is never any reason to rest on our laurels,” Sir Keir said at the forum in Nottingham.
“We have to face that and learn from it,” he said.
Labour’s Keir Mather, 25, won the contest in North Yorkshire on Thursday, beating a majority of 20,137 to become the youngest incumbent MP.
But the Conservatives have clung to the former prime minister’s Uxbridge seat, sparking debates over green policy on both sides.
Sir Keir told the BBC the Ulez plan had cost Labor in victory – but Mr Khan defended the measure as “the right one”.
Mr Tuckwell, the winning candidate, said the “damaging and costly Ulez policy” had cost Labor its seat.
Some right-wing Conservatives say a pullback from some green policies will be popular with voters, at a time when families are feeling the pressure on the cost of living.
Tory MP Craig Mackinlay, president of the Net Zero Scrutiny Group, has proposed delaying a plan to ban the sale of new diesel and petrol cars, pushing it back “at least” by five years to 2035.
Downing Street sources say there are no plans to change the climate targets – but Chancellor Rishi Sunak will try to separate his party from Labor in the coming months.
As the major parties consider the by-election results, former climate minister Lord Ian Duncan, a Conservative, warned that if Sir Keir and Rishi Sunak did not put politics aside and agree on a common approach to climate change, people would face “serious challenges”.
Lord Duncan, who served as parliamentary secretary on climate change from July 2019 to February 2020, said a “bipartisan approach” from both sides was needed to “support” common climate policies.
Politicians can win votes in the short term by moving away from aggressive climate policies – “but in the medium term, I’m not even talking about the long term, there will be serious challenges and changes to our climate that will affect people in their daily lives,” he said.
However, speaking on BBC Radio 4’s Today programme, Lord Duncan said the challenge was to ensure climate policy did not penalize those “beyond their ability to pay”.
Referring to greener technology, such as new gas boilers, he said: “We have to make sure it’s a transition and it works for everyone.”
“No one is left behind or impoverished by these policies,” he said, “otherwise that would be a problem for democracy.”