Shrewsbury town councillors almost unanimously backed a motion for the £87 million road to be rejected in favour of focusing on a sustainable transport plan for the town on Monday evening.
Now anti-relief road campaigners Better Transport Shrewsbury have said the application is fraught with errors and incomplete surveys which leave it "vulnerable" to a legal challenge.
Mike Streetly, spokesman for the group, said: "This is a truly shocking result and a wake-up call for Shropshire Council. It is extraordinary that a road that is explicitly supposed to offer Shrewsbury 'relief' has been rejected by the town council on the grounds that it offers no relief at all.
"After the recent election results and with over 3,000 objections on the council’s planning portal, it is clear that people in the town don’t think the road is fit for purpose. It is not necessary and, as Councillor Alan Mosley made clear, the Big Town Plan is not reliant on it.
"The big question now is whether or not Shropshire Council is willing to pause and reflect on what this momentous vote means, or whether they will ride roughshod over Shrewsbury’s wishes.
"We have already seen Oswestry Town Council and several parishes around the county vote against the road, while both the Shropshire Climate Action Partnership and the South Shropshire Climate Action Group oppose it on environmental grounds. If Shropshire Council wants to be ‘green’ – as new leader Lezley Picton claims – there is only one course of action left."
Mr Streetly added: "The planning application is not fit for purpose and is very vulnerable to a legal challenge or being thrown out at public inquiry. We urge Shropshire Council to take a breath and reconsider instead of throwing good money after bad on this scheme.
"Before you buy a phone or a fridge, you do your research to find the model that’s best for you. An £87 million road project is no different. Shropshire needs to follow the example of Hereford Council, which paused and reviewed its bypass and thoroughly assessed a range of alternate transport solutions for comparison. Shrewsbury doesn’t want or need this 40-year-old road plan. Our town deserves a transport package fit for the 21st Century."