Image copyright PA Image caption Ending the toll will result in higher levels of congestion, a study says

The abolition of the Severn tolls is being used to force the Welsh Government to build the M4 Relief Road, a Labour AM has claimed.

Lee Waters raised concerns after a UK government study claimed it may lead to six million more vehicles a year.

He said UK ministers were "unleashing" extra traffic and trying to "dictate" transport policy in Wales.

The UK Government said the abolition on 17 December would deliver a £1bn boost to Wales' economy.

The study, released under the Freedom of Information Act, suggests that by 2022 more than 24 million vehicles every year would use the crossings westbound, compared to 18 million if the tolls were kept roughly the same.

The Welsh Government has supported building the M4 Relief Road around Newport, but no formal decision has been taken while it awaits the outcome of a public inquiry.

Despite this, the Whitehall analysis assumes the road will be built by 2022 and claims there will be little impact around Newport.

The traffic modelling work was done after the decision was made to abolish the tolls, which was a manifesto commitment by the Conservatives in the 2017 general election.

The work estimated that by 2022 traffic on the two crossings would increase by 42% from 2014 levels - compared to 4% if tolls were kept slightly lower than the current level.

"Removing the tolls would result in higher overall levels of traffic and congestion on the wider strategic and urban road network," it said.

But the analysis said that beyond the M4 between junctions 20 and junction 23 "the impact of traffic flows would be more marginal," and that road upgrades could be needed even without the end of the tolls.

"As such, the effect of removing the tolls would act to hasten the requirement for upgrades to the network in the vicinity of the crossings, rather than to create additional congestion issues that would not otherwise have occurred", it said.

Image copyright Severn River Crossing PLC Image caption The study estimates as many as six million extra journeys across the two crossings each year

Every party in the assembly backed scrapping tolls in vote in 2016, although the matter is handled in London and not Cardiff.

Llanelli AM Lee Waters, an opponent of the M4 Relief Road, said: "It's clear from this the UK government are using the Severn Bridge tolls to try and force the Welsh Government's hand to build a new motorway.

"First they tried to bribe us with borrowing powers if only we used it on a new road, now they admit 'the effect of removing the tolls would act to hasten the requirement for upgrades to the network'.

"In all their modelling they assume that a new motorway will be built, but even if it does go ahead it won't be open for five years and in the meantime they are unleashing extra traffic onto already congested roads.

"This is at best irresponsible, at worst a deliberate attempt to dictate transport policy to Wales."

Image copyright Getty Images Image caption People had a chance to walk along the original Severn Bridge after it was opened in 1966

Welsh Secretary Alun Cairns said it was "not the case at all" that UK ministers were trying to dictate Welsh Government transport policy.

"There's been a call from the business community, from people living around Newport, from people across the whole of South Wales to build a new motorway around Newport," the Tory MP said.

Mr Cairns said he wished "the Welsh Government would get on" with building the new motorway.

A Wales Office source said that the modelling was conducted with the co-operation of the Welsh Government.

The Department for Transport said newer "more comprehensive" modelling work has since been done, and said that it shows that by 2019 there is expected to be a 23% growth in traffic.

It said Highways England and the department had been working with councils and others to prepare for the impact of removing the toll.

One of the Labour contenders to succeed First Minister Carwyn Jones, Vaughan Gething, said the Welsh Government "should demonstrate that Wales is truly open for business" and build "an M4 relief road".

"This is not a local choice. It is a major strategic choice and I am determined that we will act and go ahead," he said.

A UK Government spokeswoman said: "The prime minister and Secretary of State for Wales are committed to scrapping tolls on the Prince of Wales Bridge and Severn Crossing in a matter of weeks, strengthening south Wales' economic potential and connecting communities on both sides of the border."

A Welsh Government spokesman said: "We have been liaising with Highways England and a joint report on modelling the effect of removal of the Severn Crossing tolls is due to be published shortly."