A sustainable network of additional rail stations, rapid bus routes and cycle corridors should be built in south-east Wales to replace the scrapped M4 motorway extension, a government commission has said. The measures to relieve extreme congestion between Cardiff and the Severn Bridge could cost just half the price of the abandoned £1.4bn road scheme and would provide commuters with genuine options, according to the commission. The Welsh government welcomed the report but said it would require the UK government to “play its part” in funding rail works. Plans to build the M4 extension – a six-lane, 14-mile additional stretch of motorway bypassing Newport – were overturned by the Welsh government in 2019. Ministers cited the vast environmental damage that roadbuilding would cause to the Gwent Levels, the wetlands south of Newport that are home to rare birds, plants and insects. The UK prime minister, Boris Johnson, has angered Welsh politicians with suggestions that he would intervene and build the road scheme regardless. New powers in the internal markets bill could potentially allow Whitehall to overrule devolved governments on roadbuilding. The national infrastructure strategy published on Wednesday stated that the UK government would “make major policy interventions nationwide” should the legislation pass, with “concurrent powers to invest in infrastructure”. Costing a total of between £590m and £840m, the proposals include upgrading rail lines to increase speeds and allow frequent stopping services at six new rail stations between Newport and Cardiff, with pedestrian and cyclists given priority on station approach roads. Rapid bus routes would also be built, as well as a properly mapped network of commuter cycle corridors. Terence Burns, the chair of the south-east Wales transport commission, said parts of the scheme could be fast-tracked although the rail upgrades would probably take five to 10 years to be delivered. But he cautioned: “This is a network – you’ve got to take this as a whole. Individual interventions don’t work. “The pressure to allocate more of the roads to the car has been very strong over the years because of the congestion, because people didn’t have alternatives. We hope there is much greater chance of being able to persuade people of the bus priority.” A workplace parking levy has also been proposed, which Lord Burns said would be “one way of changing people’s preferences”, but he said: “We want to give people options, and in way that will be helpful to people and make their journeys more satisfactory.” Burns said he hoped that despite Johnson’s threats to overrule Wales on the M4, the prime minister would see the greener proposals as “tailor-made for his plans for connected nations”. Previous works in south Wales were abruptly cancelled from Westminster on cost grounds. The Welsh minister for the economy and transport, Ken Skates, said: “Getting more people out of their cars and onto public transport is critical to our future and this report shows how that approach can relieve congestion.” The proposals have been shaped by pioneering legislation in Wales protecting the needs of future generations, including tackling the climate emergency. The future generations commissioner for Wales, Sophie Howe, said it could be “a landmark moment … and testament to the change that the act is bringing about”, adding: “This must be the first of many redesigns of how we build the communities of the future with transport which is clean, affordable and accessible.”