SCOTLAND The London car bomb terrorists deliberately placed the second vehicle to catch rescuers attending the injured from the first explosion, Scotland on Sunday can reveal.
The senior security source also said the primitive gas and petrol devices were most likely the work of determined terrorists struggling - because of the security crackdown - to get their hands on the ingredients needed to create high explosives.
A security source said: "Make no mistake, if the people behind these bombs could have got their hands on high explosive then they would have used that.
"However, following on from recent high-profile court cases and obviously, the 7/7 attacks, the authorities have had a major crackdown on obtaining the necessary ingredients to make such devices. And while this has had major benefits as far as law enforcement is concerned, it has not put off the bombers - they have just changed their methods."
He added: "If either of these devices had been detonated, the resulting effects would have been devastating.
"Both of these bombs were designed to kill as many people as possible, and the addition of the nails means that even those who survived would have suffered dreadful injuries.
"The bombers knew they were not able to get their hands on high explosive or fertiliser because this would have alerted the authorities, and so they went for whatever was to hand and easy to obtain. Hence the gas canisters, the nails and the petrol."
The second bomb was only discovered when the car, also a Mercedes, was taken to an underground lot after being removed for being illegally parked. Staff there called police when they were overcome by a smell of petrol coming from the vehicle.
The make-up of the bombs led some initially to believe it was just a crude attempt to maim and injure, but this was disputed by the expert.
He added: "It may have looked simple but it was the best they could do and was by no means amateurish in its attempts.
"This is a classic situation which the armed forces and people of Iraq face every single day across the country. However, it is the first time that such a device has been used on a British street.
"While Haymarket itself is quite a wide street, the position of the car, which was parked directly in front of the entrance to Tiger Tiger and an alleyway, meant the blast would have been funnelled right into the club and all the people coming out.
"Make no mistake, these people knew what they were doing."
The second car bomb had been left on Park Lane, a busy thoroughfare around the clock.
The expert said: "The car was parked far enough away not to immediately raise suspicion but close enough to be in the vicinity of ambulances, fire engines and police cars attending the first blast.
"Also, because it is a main road, there would have been a very strong possibility that other passers-by would have been caught up in the chaos too when that one detonated.
"It is only by the grace of God that both of these devices were discovered, but the authorities know that there will almost certainly be others."