A dangerous defect indicates a direct and immediate risk to road safety or a serious environmental impact. It causes an MOT failure and requires repair before the vehicle is driven.
Does an old MOT certificate still help?
No. GOV.UK only allows a failed vehicle to be taken away when the existing MOT is valid and no dangerous problem was listed. An unexpired certificate does not override a dangerous result.
Possible penalties
GOV.UK states that driving a vehicle that failed its MOT because of a dangerous problem can result in a fine of up to £2,500, a driving ban and three penalty points.
How to move the vehicle
Ask whether the test centre can complete the repair. Otherwise arrange suitable recovery to a repairer. Do not assume that booking a repair appointment makes it lawful or safe to drive a vehicle with a recorded dangerous defect.
Official source
Read GOV.UK: MOT test result.