Labour’s defeated candidate in the Uxbridge by-election has launched a vicious attack on Sadiq Khan’s controversial expansion of London’s Ultra Low Emission Zone (ULEZ).

Danny Beales said the policy had “cut us off at the knees” and handed the seat to the Tories.

In a surprise appearance at Labour’s National Policy Forum (NPF) in Nottingham this morning, he declared: “ULEZ is bad policy. It must be rethought.”

His comments are a further sign of the bitter Labour civil war that has erupted since Conservative candidate Steve Tuckwell defied the odds to win Uxbridge and South Ruislip by just 495 votes.

The result was a major boost for Rishi Sunak on the same night the Tories suffered seismic losses to the Lib Dems in Somerton and Frome and to Labour in Selby and Ainsty.

Khan, the Labour mayor of London, has insisted he will not scrap the ULEZ expansion, which takes place next month and will see drivers of cars that don’t meet strict environmental standards charged £12.50 a day.

But Beales told the NPF: “Our relentless focus on the cost of living hammering voters across the country should have been enough to win my home seat. But it wasn’t.

“Because – let me be frank – a single policy cut us off at the knees. This isn’t complicated. You cannot tell working people you are laser-focused on the cost of living, on the difficulties facing them, on making life easier and then also penalise them, simply for driving their car to work.

“ULEZ is bad policy. It must be rethought.”

He added: “There were people in Uxbridge desperate for change, sick of the Tories, complimentary about our changed party, about our leadership, about our plans.

“But a single policy – one that felt like a grotesque unfairness to many who might otherwise have voted for us – acted as a dead-weight, one that we were forced to trudge around with on our backs, all day, every day, from one door to another.”

Beales said Labour must use the NPF gathering to learn the lessons of the Uxbridge defeat and focus its policies on helping ordinary people currently struggling with the cost of living.

He said: “We can continue to drive our party back into the arms of working people, as we are doing under Keir’s leadership, by focusing entirely on their priorities, their needs and their desires.

“Or we can spend this weekend focused on the hobby horses, the ideological obsessions and – ultimately – the damaging policies that cost us dear in Uxbridge.”

Keir Starmer yesterday called on Khan to “reflect” on the part ULEZ played in Labour’s by-election defeat.

But the mayor vowed to press on with the controversial policy, saying clean air is a “human right, not a privilege”.