More than £15m was spent on the planned M4 relief road before the project was scrapped, Welsh Conservatives have learned.
Figures obtained by South Wales East AM William Graham reveal that between 1998 and 2008 some £15.57m was spent on the design development of the route between Magor and Castleton near Newport.
Of that figure some £13.5m was paid to technical advisors, a letter from Transport Minister Ieuan Wyn Jones says.
The project, which business groups identify as critical to the South Wales economy, was ditched by the Labour-Plaid Cymru Assembly Government last month.
At the time Welsh Conservatives described the decision to abandon the project as limiting the prospects for economic growth in the region.
The figures obtained by William Graham AM reveal that in the last two years alone more than £8.4mn was spent on the scheme, including nearly £700,000 on surveys and £7.1m to technical advisors.
Around half of the £15mn spent on the scheme was spent in the last two years.
William Graham AM said:
"The proposed M4 relief road was one of two major strategic requirements for the transport network in south east Wales.
"It is clear that even as recently as last year the Assembly Government was committed to its development and was spending millions on turning it into a reality.
"While we accept that major projects such as this inevitably require careful planning over a number of years £15mn has effectively been wasted by the Assembly Government's decision to abandon it.
"We continue to share the business community's fears that prospects for economic growth will be limited by this strategic bottleneck.
"Plaid Cymru's transport minister needs to explain at what point he decided to pull the plug on the M4 relief road.
"Was this decision taken in the last few weeks or months? Or was he being advised to scrap it last year at the same time as spending on its development was higher than all but one of the previous 10 years?"
Shadow Transport Minister David Melding AM said:
"Such considerable expenditure on feasibility planning for this scheme demonstrates that the road was accepted by the Assembly Government and its technical advisers as a crucial project.
"The decision not to proceed represents a major U-turn by the government and means millions of pounds have been lost and taxpayers will have nothing to show for it."