Council bosses have been accused of "moving the goalposts" on Bath's clean air zone after drivers who checked their vehicles were exempt were hit with charges.

Tradespeople say the blunder has led to a spike in demand for grants and interest-free loans for upgrades – with fears the funding will run out – but the pandemic has caused a national shortage of vans.

Bath and North East Somerset Council has blamed the government for the confusion, saying it only learned after the zone launched on March 15 that the vehicle checker had been updated two weeks earlier.

The authority has contacted affected drivers, waived charges and given them time to consider their options.

Adrian Jennings, a tiler, said: "Right from the start I've been using the vehicle checker and my van wasn't chargeable. Suddenly the council moved the goalposts and my van was chargeable.

"They want me to exchange my vehicle. I have to go into £25,000 of debt in a pandemic, and there's going to be a recession.

"I can't do anything until they give me a tracker. You can't get trackers because there aren't enough of them."

The council uses telematic trackers to check if drivers are eligible for support, in the form of grants worth up to £4,500 for vans or interest-free finance. To be eligible for the scheme, drivers must have a chargeable, non-compliant vehicle that travels an average of two or more times a week in Bath's clean air zone.

Mr Jennings questioned why trackers are needed at all when the council has paid millions to install a camera network that can read number plates, but the authority said its use is strictly controlled.

"Manufacturers are saying you can't get a van until 2022 – they can't finish them because of Covid," he said. "This is what the council don't realise.

"It's a complete con. Everyone has been driven out of Bath."

Anyone who fails to pay the £9 a day charge for vans, taxis and private hire vehicles or £100 sum for buses, lorries and coaches faces a £120 fine. Private cars are not charged.

Mr Jennings claimed the clean air zone had resulted in new rat runs through Weston village and past Ralph Allen School as drivers try to avoid the charge, which he said was also catching out residents in large vehicles like Land Rovers and VW Transporters.

Plasterer Glyn Hitchcock said after months of waiting he managed to get a tracker fitted and drove around with it for 60 days as required, but when he handed it back it was faulty so he had to start all over again.

He said the costs of the clean air zone charges would inevitably be passed on to customers.

A plasterer who had no problems getting a grant towards a van but asked not to be named said the council should have waited so businesses hit by the pandemic were not also struggling with clean air zone charges.

B&NES Council said it was only made aware that the government had updated its vehicle checker after the zone launched. The change affected a "small group of vehicles" that previously incorrectly identified as being compliant.

The council said: "We have been writing to the owners of these vehicles who failed to pay for their journeys. We have waived all outstanding charges since 15 March, and no fines have been issued.

"We are also exempting these vehicles from charges for a further 10 days so that owners have some time to consider their options. We have interest free finance and grants of up to £4,500 per van available to help drivers replace non-compliant vehicles.

"We put an ambitious bid to government for £11.2million and we have so far been awarded £9.4 million to support around 1,900 vehicle upgrades of the most polluting vehicles among eligible candidates.

The remaining funding would only allow 444 more motorists to get a £4,500 grant but the council said it was working closely with applicants and central government to ensure that as many vehicles as possible are upgraded.

It said despite engaging with some 9,500 firms in the second half of 2020 it had a surge of interest in support two weeks either side of the launch. The council has secured government funding for extra staff to help speed up the application process.

It has received a handful of reports about increases in traffic along certain roads, so is reviewing the modelling and if appropriate undertaking traffic monitoring, but said the situation has been complicated by the lockdown restrictions being lifted.

On average during the first month of operation, around 31,585 unique vehicles a day were driving in the zone, and around seven per cent were chargeable.

In the zone's first two weeks more than 3,700 drivers failed to pay for their journeys – nearly one in eight.

The council said: "Drivers of non-compliant vehicles that fail to pay for their journeys in the zone are receiving notice letters for each day they fail to pay, but nobody has yet been asked to pay the fine of £120.

"We are only asking drivers to pay the outstanding charge in the first few instances. This is for a short period only while everyone gets used to this new scheme."

Defra said it made the update after improving its data accuracy on which vehicles were compliant with clean air zone emission standards.

Petrol vans with a gross vehicle weight over 1,330kg registered in 2006 and diesel vans with a gross vehicle weight over 1,330kg registered between September 1, 2015, and September 1, 2016 are now subject to a charge.

A Defra spokesperson said: "It is important that the most polluting vehicles are charged to enter clean air zones to reduce roadside nitrogen dioxide concentrations to legal limits in the shortest possible time.

"There is financial support available to non-compliant vehicle owners to upgrade their vehicles to meet the emissions standards of the Bath clean air zone."