THE terrorists who attempted to bomb central London last week 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.
Yesterday, a huge police manhunt was under way for the terrorists responsible as forensic experts continued to examine the vehicles involved for clues.
The attack was thwarted after fumes were spotted leaking from the first vehicle, parked outside the packed Tiger Tiger nightclub in London's West End in the early hours of Friday morning.
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 whilst 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 alert over the first device was raised only after an ambulance crew, dealing with a drunk girl outside Tiger Tiger, spotted smoke billowing from the Mercedes parked just feet away.
Police called to the scene evacuated the area after noticing a mobile phone on the back seat which is believed was to be used to detonate the device.
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.
"Whilst 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 ambulance, 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."
Police search for London four
POLICE investigating two attempted car bomb attacks in London are keen to rule out of their inquiries four men who absconded from control orders imposed under anti-terror laws.
The men - three of them believed to be from North Africa and described by former Home Secretary John Reid as "dangerous" - have been on the run for some months.
Algerian brothers Lamine and Ibrahim Adam, and Cerie Bullivant have been missing since May. A fourth man, Zeeshan Siddiqui, a former London Underground worker, went missing last year.
On Friday, two car bombs - packed with petrol, gas cannisters and nails - were found in London, prompting new fears that the UK is about to face a fresh wave of terror attacks.
Police believe the bombs to have been part of an "al-Quaeda inspired" attack on London nightclubs, which could have killed large numbers of revellers. The design of the devices left near Tiger Tiger, say experts, sis imilar to bombs used by insurgents in Iraq.
Police sources have said they are keen to speak to the four men in connection with the London car bomb attempt to rule them out of their inquiries.
When the two Adams and Bullivant absconded, Reid said they were not a "direct threat" to people in the UK but added they were dangerous.
Lord Carlile, who has reviewed British anti-terrorist laws, said of the same three: "These...men were the subject of solid intelligence that they intended, not to cause damage in the UK, but that they intended to damage our national security by going as insurgents to kill British and other allied troops abroad, in Iraq for example.
Another Adams' brother, Anthony Garcia, is serving life for plotting explosions. Garcia was one of five men jailed earlier this year over a plot to target the UK with a giant fertiliser bomb.
Meanwhile, London Ambulance Service have released a statement concerning the paramedics who discovered the first car bomb in London's West End in the early hours of Friday.
The two-man crew, one aged 27 and the other 37, were also on duty on July 7, 2005, when four suicide bombers killed 52 people on the underground system.
The 37-year-old, who was not named, said: "As we pulled up outside Tiger Tiger, we came to a stop behind a Mercedes car, which was parked rather badly, about three feet from the kerb.
"We thought it was a bit odd and that led my crewmate, who was driving, to pull around the car and park just in front of it.
"As we got out and walked past the car we realised it had its lights on but the engine was off.
"I thought I saw a jet of smoke coming out from between the front seats and my crewmate also noticed there was a funny smell of gas.
"At that stage our priority was to find and help the patient we had been called to.
"Once inside, we treated a man who had suffered minor head injuries after a fall. He did not need to go to hospital so we sorted him out quite quickly and headed back outside to our ambulance.
"We can't have been inside for more than a few minutes and as soon as we got outside we smelt gas again.
"Then we saw the jet of smoke was still there so we got straight on the radio to our control room and asked them to call the police and the fire brigade.
"We just did what any ambulance crew would have done - we noticed something we thought was odd and we acted on it. I am just glad that we managed to do that before it was too late."
Russell Smith, London Ambulance Service's deputy director of operations, said he thought the actions of the crew saved "many people".
"We are immensely proud of the part our crew had to play in alerting the police to the suspect vehicle and genuinely believe that their vigilance and prompt action helped saved the lives of many people in London," he said.
"This is a great example of the part we as individuals can play in helping to keep each other safe."
Scotland Yard officers have been continuing to examine hours of CCTV footage in an attempt to identify the people who planted the cars although they have refused to comment on reports that they had already identified a "crystal clear" image of the driver of the first car.
A spokesman said: "Many lines of inquiry are being followed by detectives.
"Painstaking trawls of CCTV footage to establish the circumstances leading up to the events in the early hours of yesterday morning, and those responsible, are being carried out.
"Forensic experts are continuing examinations of the two cars which contained the devices. This is an extremely complex process."