Cars, vans and small buses will pay just £3 in tolls when the Severn crossings go into public ownership around 2018, under UK government plans.

The proposed reduction is more than previously suggested, and is less than half the current £6.70 charge for cars.

Ministers are also seeking views on the introduction of a two-way "free-flow", barrier free charging system on the crossings.

A consultation is being launched on the proposals on Friday.

Transport secretary Chris Grayling said: "The future toll prices announced today will not only guarantee the future and safety of the crossings for generations to come but will also boost the economy whilst offering the best value for money for motorists and the taxpayer."

Baroness Randerson, Liberal Democrat transport spokeswoman and a former AM, said the proposals do "not go far enough".

She said: "Why should people using this bridge to pay for its upkeep when they already pay for road repairs through the tax system, just as users of other motorways do?"

Image caption Proposals on the tolls were detailed in a letter from the UK government to AMs and Welsh MPs

The news was detailed in a letter from Welsh Secretary Alun Cairns and UK Transport Minister John Hayes to AMs and Welsh MPs.

If a two-way "free-flow" charging system is also brought in, it would see the removal of toll barriers, with cameras being used to detect cars crossing the bridges, and tolls would be charged both ways.

It could also mean a toll of £1.50 each way for cars, vans and small buses, and £5 for larger vehicles.

The letter said the consultation would set out both the advantages and disadvantages of introducing such a system, which is already in use to enforce the London congestion charge.

"We understand the importance of the crossings for both the Welsh and English economies, and that they have benefitted both Welsh and English road users for 50 years," the ministers said in the letter.

"We believe that the approach set out in our consultation provides a sustainable approach, and a good deal for both users and the taxpayer in the decades to come."

Under the plans:

Car tolls would be reduced from £6.70 to £3

Vans and small buses will also pay only £3, down from £13.40

Lorries and coaches will pay £10 instead of £20

Users of the Severn TAG cashless toll system will also see their charges reduced.

Image caption Could toll barriers be removed from the Severn bridges?

The letter said there was no intention to use the tolls for any other purpose other than to support the operation and maintenance of the bridges, and to repay the debt incurred by the UK taxpayer to fix "latent defects on the crossings".

"We can also assure you that the government will monitor toll prices closely with a view to further reductions if possible in future," it said.

Welsh Transport Secretary Ken Skates reiterated the Welsh Government's calls for the tolls to be scrapped once the Severn bridges revert to public ownership.

"We will continue to lobby the UK government for this," he said.