Plans to convert more hard shoulders into permanent driving lanes to ease congestion should be scrapped amid safety fears, MPs have recommended.
The Government intends to expand motorway capacity by converting hundreds of miles of hard shoulder into permanent lanes, known as 'All lane running'.
But an inquiry by MPs on the Commons Transport Select Committee concluded the plan is too dangerous and has not been properly considered.
Current 'smart motorway' schemes have only used the hard shoulder at peak times or to deal with congestion.
The new plans would permanently convert hard shoulders into traffic lanes to increase capacity, with a 60% increase in motorway traffic forecasted by 2040.
Opening up hard shoulders to traffic is seen as a cheaper and less disruptive alternative to widening motorways with extra lanes.
But the report published on Thursday by the Transport Committee warns ministers to call a halt to 'All lane running' schemes while major safety concerns exist.
The MPs did not agree with the Government that future schemes are an "incremental change" and a logical extension of previous schemes, where hard shoulders were used during rush-hour congestion.
Chair of the Transport Select Committee, Louise Ellman MP, said: "The permanent removal of the hard shoulder is a dramatic change.
"All kinds of drivers, including the emergency services, are genuinely concerned about the risk this presents.
"It is undeniable that we need to find ways of dealing with traffic growth on the strategic network. But All Lane Running does not appear to us to be the safe, incremental change the Department wants us to think it is."
MPs say the scheme on the M42 has a good safety track record but newer schemes are less safe and public awareness about how and when to use the hard shoulder is insufficient.
Plans are in place to permanently convert the hard shoulder into a traffic lane on around 300 miles of motorway, with 30 schemes proposed.
RAC chief engineer David Bizley said: "Whilst supporting smart motorways as a cost-effective and relatively rapid way of increasing motorway capacity, the RAC has repeatedly expressed concerns about the latest design which turns the hard shoulder on motorways into a permanent running lane.
"The safety of motorists must come first and therefore new designs need to be trialled for sufficiently long to demonstrate their safety before they are introduced more widely."
Edmund King, the AA president, added: "Breaking down on a motorway in a live running lane is every driver's worst fear.
"Right from the outset the AA raised substantive safety concerns, also voiced by our members, over the dangers of breaking down on a motorway without a hard shoulder or with an inadequate number and size of lay-bys.
"Whilst we need to increase capacity and reduce congestion we must ensure that we are not cutting corners which compromise safety just to reduce costs."
A Department for Transport spokesman said: "All Lane Running roads are designed to be as safe as ordinary motorways.
"In the two All Lane Running sections on the M25, accidents were down 17% and casualty rates fell by 21% in the first year.
"As the committee recognise, the busiest journey times have almost halved, and overall journey times are more reliable and predictable than before.
"We will be considering all the Transport Select Committee's findings carefully and responding shortly."