More than 1,400 vehicles have been seized from drivers who have persistently ignored fines relating to London’s Ulez clean air zone, Transport for London has revealed, with more than £25m being recouped by bailiffs.

Bailiffs working on TfL’s behalf seized 1,429 vehicles in the last year from drivers who had repeatedly ignored penalty charge notices, with £710,000 being raised from the sale of nearly 800 of these cars.

The figures, which cover the 12 months up to the end of July, come a year after the mayor of London, Sadiq Khan, extended the Ulez to cover all 32 London boroughs from a previous zone that covered the area between the capital’s north and south circular roads.

Vehicles that do not meet certain emissions standards and are caught using roads in the zone must pay a £12.50 daily charge, or a fine of up to £180 for non-payment.

If drivers do not pay this penalty it is registered as an unpaid debt and an order is made for recovery. If it remains unpaid, a warrant is issued which allows bailiffs to recover the outstanding debt.

The latest figures from TfL, covering June, showed that the average number of vehicles charged is 42,555, with 6,004 penalty notice charges (PCNs) being issued.

TfL said that in the past 12 months bailiffs had recouped £25.6m from those who refused to pay penalty charge notices. This included one driver who was forced to settle a balance of £16,000 after 45 warrants were issued against them. In another case, a vehicle was seized to pay off an outstanding balance after the driver ignored 10 warrants.

TfL said it is owed more than £370m in unpaid Ulez penalty charge notices since the scheme expanded last year but added that it believed a significant amount of this debt would be recovered through law enforcement.

TfL has hailed the Ulez expansion as a success, pointing to research showing that it has reduced levels of harmful air pollutants significantly since its introduction, as well as helping with the climate emergency by cutting London’s emissions.

However, the scheme has also faced a strong backlash from some, including owners of non-compliant vehicles refusing to pay fines and others vandalising the cameras that police the zone.

TfL said a significant amount of debt remained outstanding and it was now tripling the number of investigations staff to help enforcement agencies target repeat offenders.

While the compliance of a vehicle is based on declared emissions rather than its age, a rule of thumb is that it affects diesels made before 2015 and petrol cars before 2006.

Last month, TfL was forced to refund drivers in Chingford, east London, after its camera had become misaligned and incorrectly charged vehicles outside the Ulez boundary.

In January, the Guardian revealed that hundreds of thousands of EU citizens could have been wrongly fined for driving in the Ulez zone, with five EU countries accusing TfL of illegally obtaining names and addresses of citizens in order to issue fines.

Alex Williams, TfL’s chief customer and strategy officer, said: “The most recent data shows that on average, over 96% of vehicles seen driving in the Ulez are compliant.

“We want to send a clear message to vehicle owners that if you receive a penalty charge for driving in the zone, you should not ignore it. Your penalty will progress to enforcement agents to recover the fines that you owe, and there is a risk that your vehicle and other items of property will be removed.”