The MOT exemption – introduced following the coronavirus outbreak – has increased the number of vehicles which must pass the test in September to stay on the road.
Cars, motorcycles and light vans due an MOT between March 30 and July 31 2020 were given a six-month extension due to the nationwide lockdown.
The AA say some 20% of 14,688 UK motorists it surveyed said they took advantage of the policy – indicating that more than 5.5 million cars had a delayed MOT.
Many drivers eventually had their vehicles tested in September 2020, meaning they must book another test for next month.
They will be competing for slots with the owners of the 339,000 new cars bought in September 2018 which will be due their first MOT.
AA head of roads policy Jack Cousens said: "With more than 5.5 million cars deferring an MOT it is crucial that they are tested to ensure they are safe to use.
"MOT centres are already feeling the strain and with 'Super September' looming, savvy drivers can get ahead of the game. Don't delay, book today.
"Usually people leave booking their MOT to the last possible moment. Drivers won't have that luxury this time."
Drivers can be fined up to £1,000 for using a vehicle without a valid MOT.
Vehicles must have an MOT on the third anniversary of their registration, and then every 12 months.
A number of parts such as lights, seatbelts, tyres and brakes are checked to ensure they meet legal standards.