Millions more vehicles will cross the Severn Bridge between England and Wales every year after tolls come to an end in December, a government study claims.
The change will force the Welsh government to press on with plans to build a new M4 relief road to absorb the extra traffic, a Labour AM claims.
Lee Waters, who represents Llanelli, accused the Government in Whitehall of trying to dictate transport policy by “unleashing” extra traffic on Wales.
The government study, released under the Freedom of Information Act, predicted a rise in vehicle movements into Wales to more than 24 million a year by 2022.
This would represent an increase of six million vehicles annually and is based on Government assumptions that the new relief road would be built by then.
Ministers say the abolition of tolls on December 17 this year is a positive step for Wales, delivering an estimated boost of £1 billion to its economy.
Abolition of the tolls was a Conservative manifesto pledge at the last election and the traffic report was commissioned to gauge its effect.
The authors estimated that traffic on both Severn crossings would increase by 42 per cent by 2022 from 2014 numbers if tolls were scrapped.
They said there would be “marginal” impact on the M4 between junctions 20 and 23 and that motorway upgrades would be necessary even if tolls remained.
The scrapping of tolls attracted support from all parties in the Welsh Assembly when it was put to a vote in 2016.
Mr Waters, who opposes the relief road, said: “It’s clear the UK government are using (Severn) tolls to try and force the Welsh Government’s hand to build a new motorway.
“In all their modelling they assume a new motorway will be built, but if it does go ahead it won’t be open for five years.
“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.”
The Welsh Government supports the new M4 relief road around Newport being built but is awaiting the findings of a public inquiry before making a decision.
The UK Government report assumes the road will be built within four years and that the impact around Newport will be minimal.
The Department for Transport said it had been working with Highways England, local councils and other agencies to prepare for the removal of tolls on the bridge.