The Welsh government has increased the circumstances under which charging schemes could be introduced on trunk roads.

Charges may now be introduced on roads in order to reduce or limit air pollution, as part of the amendments to the Transport Act 2000.

It comes after the default speed limit in built-up areas was reduced to 20mph in September on roads in Wales.

Similar schemes have faced public objections, including London's ultra-low emissions zone.

The Welsh government's climate change minister, Julie James, said the initiatives had been introduced "as a necessity to make a step change improvement in air quality".

Ms James said the government had "made good progress" and had so far introduced non-charging mitigations.

But ongoing air pollution problems "may require more substantial interventions" in future.

The minister insisted there were "no current plans" to introduce charges on trunk roads.

Instead, clean air zones would be "precautionary retained measures that could be introduced on certain roads if other measures prove insufficient".

Nearly 7,000 people have signed a Senedd petition calling for all further planning on road charging in Wales to be stopped.

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The Welsh Conservatives' shadow climate change minister, Janet Finch-Saunders, said she did not believe ministers' road-charging powers should be expanded.

"The introduction of trunk road charging schemes could actually result in more people using local authority-managed roads, compounding congestion in sometimes difficult-to-navigate communities," she said.

"Giving people a penalty for driving when there is no feasible alternative option is unreasonable and unjust."