Twelve double-decker coaches withdrawn from service following the M25 crash have passed safety checks.
Manufacturers Neoman said the coaches were thoroughly examined and there should be no concerns among passengers regarding their safety.
National Express removed its fleet of Neoplan Skyliners after one of them overturned, killing two on board.
The coach company said the fleet will not be returned to service until it had completed its own investigations.
The Neoman Bus group said in a statement: "We believe there should be no concerns among passengers regarding their safety due to the technical performance of these vehicles.
"Neoplan double-deck vehicles operating in the National Express fleet have been examined thoroughly overnight.
"As expected, all have passed these safety checks and are available for immediate return to service as and when the operator wishes."
National Express is conducting its own inquiry and said the Skyliners would not return to service until the investigation was complete.
The Neoplan Skyliner has, by law, been fitted with a speed limiter restricting it to 62mph.
All double-decker vehicles undergo Department for Transport safety tests to show they can tilt 28 degrees with the top deck full and the bottom deck empty, without tipping over.
Simon Posner, from the Confederation of Passenger Transport, said: "Coach travel is by far the safest way to travel on the roads today, every figure will show you that.
"Also, there is no evidence showing double-deck vehicles are more likely to cause accidents."
The coach involved in Wednesday's incident is being examined by accident investigators.
The 40-year-old driver who was arrested on suspicion of causing death by dangerous driving has been released on bail.
Twenty-three people remain in hospital, some of whom have lost limbs, following the crash on a motorway slip road near Heathrow.
Two remain in a critical condition and 10 are said to be serious.
A man and a woman died. The woman was later named as Chris Toner, 76, from Monifieth, near Dundee.
But police have yet to identify the man, who is described as East Asian in appearance, in his mid-20s and between 5ft 6in and 5ft 8in tall.