Sir Keir Starmer said Labour will be “more ambitious” than the Conservatives’ plans for Tata steelworks in Port Talbot.
The UK government has promised Port Talbot £500m to move to greener steel, but it could see thousands lose their jobs.
The Labour party leader said he wanted a “plan that preserves the jobs we’ve got”.
He was on a visit to Port Talbot steelworks.
However Sir Keir did not deny that, once the steelworks transitions to green energy, there would be fewer people working there.
“My biggest concern at the moment is that we have a plan that ends what we’ve got, has an impression of what we might have but hasn’t done the hard yards of the bridge between the two,” he said.
In September the UK government announced extra investment for the Port Talbot steelworks for the installation of new electric arc furnaces.
At the time Tata warned there would be a “transition period including potential deep restructuring” at the plant.
Tata Steel employs about 8,000 people in the UK, 4,000 of those in Port Talbot.
Unions previously said the move to the new less labour-intensive furnaces could lead to thousands of job losses.
“We would be more ambitious”, he said.
“We want to drive up the demand for steel. We need to go to clean power by 2030.
“That’s going to require a lot more steel.
“We want to ensure there’s a plan that preserves the jobs that we’ve got, the experience that we’ve got.
“Combine that with a bridge to the future so that the future generation of josb is here as well.”
Asked if politicians should be honest and say there will be fewer people needed with green steel, he said: “I think transition is always difficult because there’s always anxiety about protecting the jobs that are already in existence. That is completely understandable.”
Sir Keir visited the steelworks with Labour First Minister Mark Drakeford, who did not speak to the media at the event.
The UK Labour leader declined to answer whether he would commit to providing HS2 funding to the Welsh government, or look at the Cardiff administration’s funding model, following last week’s announcement of cuts to help support the NHS and train services.
Instead he said a future UK Labour government would work with the Welsh government “on a rules-based basis, so that we can co-ordinate”.