Better Shrewsbury Transport (BeST) said that a radio question and answer session earlier this week further fuelled concerns that the £81 million project will be a financial disaster.
Mike Streetly, a spokesman for BeST, said: "After listening to the interview we were left with grave concerns about the spiralling cost of this project. Councillor Picton talked about the severe budgetary pressures that the council is under and described spending £30m on Shirehall as 'throwing good money after bad'.
"When it comes to the NWRR, though, she remained determined to carry on regardless, even though the project risks plunging Shropshire into a black hole of debt."
Shropshire Council has claimed that it can fund the £28m it needs to build the road by selling assets, including greenfield land to developers. BeST say that there is still no transparency on what exactly will be sold.
Mr Streetly added: "Shropshire Council has a disastrous track record when it comes to spending taxpayer money. Despite teetering on the edge of bankruptcy, the council is committed to spending at least £28m on the road. The only way it can do this is through a fire sale of assets.
"At a time when people across Shropshire are paying more council tax for reduced services, how can the council justify flogging the family silver to pay for a road for Shrewsbury that Shrewsbury Town Council doesn’t even want? It is an insult to people in towns like Bridgnorth and Oswestry and elsewhere who are seeing essential services slashed as their council tax hiked up."
Campaigners say they are also waiting for an explanation of how the road will be funded if it goes over budget. With the government grant for building the road capped at £54m, the council will have to fund any overspend itself, possibly through loans.
"We believe the biggest danger is if the road goes over budget," said Mr Streetly. "Our research suggests that the council will need to find another £30 to £40m, not least because inflation in the construction industry is reported to be running at over 20 per cent. Every month of delay costs the council another million pounds. What exactly is Councillor Picton’s plan to deal with this? We still don’t know.
"If Shropshire Council takes out a costly loan, it will only cause further misery for Shropshire residents since the interest on it will be paid from the revenue budget which is already in 'uncharted waters'. We urge the council to reconsider before plunging itself – and all of us – into financial misery."
A Shropshire Council spokeswoman said: "We have not changed our budget provision of underwriting £19.8m for the Shrewsbury North West Relief Road and £8.7m for Oxon Link Road, against potential developer contributions towards these projects. There remains high confidence that a substantial amount of this funding can be recovered through developer funding and local capital receipts.
“We are constantly reviewing how we can deliver this project and ensuring that we minimise the risks as far as possible, so revisiting the design for the viaduct is an example, talking to the supply chains as early as possible, ensuring that we have undertaken investigation works in advance to provide as much detailed information as possible to remove uncertainty in cost pricing and allow the cost to be minimised. We are aware of the current national and international issues such as HGV drivers, energy costs and inflation, but by providing surety and the highest quality information as possible, we can then minimise any risks as far as possible.”