New MOT rules will be introduced 20th May 2018 making it harder for diesel cars to pass.

The vehicles are to be put through tougher emissions tests and faults rated in three defect categories -

• Dangerous - immediate risk to road safety / impact on environment
• Major - vehicle less safe, impacts environment, puts other road users at risk.
• Minor -  no significant effect on safety of the vehicle or impact on the environment.

Any car that has been fitted with a diesel particulate filter that give out "visible smoke of any colour" during tests will get a Major fault and also automatically fail. If the filter looks as if it's been removed or tampered (unless it can be proved it has been done so for filter cleaning) the car will also fail.

Neil Barlow, head of MOT policy for the Driver and Vehicles Standards Agency told Auto Express the new rules will "help motorists do the right thing".

He added: "We're changing the wording on the certificate. We've done a lot of research with motorists to find out what sort of information helps."

Steering is also to be looked at in the DVSA's new criteria.

A steering box leaking oil would get a Minor fault but if the oil was dripping badly it would be pushed up to Major and fail.

Reverse lights will be checked and brake discs also inspected to see if they are "significantly or obviously worn".

An RAC spokesman said they fear the changes could end up confusing motorists.

He said: "Rather than MOT failures simply being black and white, the new system creates the potential for confusion as testers will have to make a judgement as to whether faults are Dangerous, Major or Minor.