The Commons Transport Select Committee concluded the plans for all lane running would be too risky for drivers and have not been properly considered.
The RAC has also expressed “concerns” over the proposals, saying new designs need sufficiently long trials before they are introduced.
Current smart motorway schemes have used the hard shoulder only during peak times or to deal with congestion.
But under the Department for Transport plans, hard shoulders would be converted into permanent lanes to increase capacity, with motorway traffic forecasted to rise by 60% by 2040.
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.”
Plans are in place to permanently convert the hard shoulder into a traffic lane on around 300 miles of motorway, with 30 schemes proposed.
Opening up hard shoulders to traffic is seen as a cheaper and less disruptive alternative to widening motorways by building extra lanes.
But the report published on Thursday by the Transport Committee warns ministers to call a halt to conversion schemes while major safety concerns exist.
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 trialed for sufficiently long to demonstrate their safety before they are introduced more widely.”
A spokesman from the Department for Transport said the select committee’s findings would be carefully considered.
Last year the RAC welcomed a decision by MPs to review the distances between emergency rescue areas (ERAs) on all lane running smart motorways.
A survey of drivers who had broken down on all lane running stretches of the M25 carried out by the organisation suggests that only 42% knew that ERAs existed.
Little over a quarter (28%) of those questioned, the poll found, could actually see one from where they had broken down.