Motorways are not safe enough to cope with raising the speed limit from 70mph to 80mph, a report has said.
The government intends to consult on introducing an 80mph limit on English and Welsh motorways.
However England's motorways do not provide enough protection to drivers and car occupants for an increase in the limit to be considered, the report from the Road Safety Foundation (RSF) said.
It added that there would be a rapidly-rising risk of shunt crashes from the sheer volume of traffic using the motorways.
The report also said there were widespread faults in the protection given to drivers to prevent them running off the road on motorways.
The RSF also cited the problem of pedestrians on the hard shoulder, roadworks, extreme weather and spillages from vehicles.
In addition, there would be negative economic effects from a higher speed limit, including increased vehicle operating costs through higher fuel consumption; increased crashes and crash severity, resulting in raised crash costs; and the increased cost caused by delays from crashes.
The foundation said it recognised the argument that respect for the 70mph limit was poor and it did not dismiss proposals to raise the limit.
"Drivers who want to are already travelling at 80mph when they can," the report concluded.
"Economic benefits only arise if '80 means 90' and opinion surveys show no public support for that.
"However, large economic benefits arise from fixing the motorways systematically rather than raising the speed limit."
Proposals to bring in 80mph limits were first put forward in autumn 2011 by the then Transport Secretary Philip Hammond.
If a higher limit is introduced, there would still be some sections of motorway where the limit would stay at 70mph.
Roads minister Mike Penning said: "The Department for Transport is carrying out work to assess the potential economic, safety and environmental impacts of trialling 80mph speed limits on motorways where variable limits are currently in place.
This work is on-going and no final decisions have yet been taken about which stretches of motorway would be included in any proposed trial.
We plan to bring forward detailed proposals and start consultation during the next few months."
RAC technical director David Bizley said: "RAC supports the trials being done by the Department for Transport to assess the impact of an 80mph speed limit.
Safety is always the greatest concern and we are urging the Government to raise the limit only on those stretches of motorway where it is appropriate to do so.
A blanket approach is no longer viable and both road quality and safety risks have to be taken into consideration."