The builders of the bomb found in central London today would have probably been "keen amateurs", an explosives expert said.

Patio gas cylinders found by police in the light green Mercedes would have been an unlikely weapon for experienced terrorists unless they wanted to create a fireball for the cameras, Sidney Alford, founder of explosives company Alford Technologies, told Guardian Unlimited.

As a readily available combustible material, the propane gas held in such cylinders might be considered by someone unable to source high explosives.

"If you are making a bomb and you are limited in the amount of explosives you can acquire you could easily get some gas cylinders of propane to add to them. They would give a more impressive fireball on TV," he said.

"They are probably keen amateurs who could not get their hands on the real thing and do not realise the limits of what they are doing."

The Metropolitan police's head of anti-terrorism, Peter Clarke, said there was also a "significant quantity" of petrol and nails in the car, as well as the gas. A witness reported nails were lying on the floor of the car, which Mr Alford said was another indication the bomb makers were inexperienced.

"Nails could be considered as an additional way of extending the potential damage and lethal range of the device but putting them on the floor is an incompetent way of building a bomb. They would go straight into the ground," he said.

Mr Alford said that unless there were also explosives present the main impact of the device would be in the economic disruption caused by closing off the normally bustling shops, restaurants and businesses of central London.

"As the IRA knew, you do not need a real bomb to cause real havoc," he said.

Police said the "potentially viable explosive device" was made safe. Mr Clarke said that had it exploded there could have been "significant injuries or loss of life".

Bomb disposal experts, Mr Alford said, may have used a water disruptor often delivered through a "pig stick" - similar to a gun barrel - to try and separate the components of the charge without triggering an explosion.

Water could be fired at near-supersonic speeds down the stick and its force would be enough to pierce metal and destroy the bomb, but without causing a spark.

It is believed to be unprecedented for gas canisters to be used for a bomb in the UK. They have sometimes been used elsewhere in the world as bomb casings - opened up and packed with explosives - or used for firing mortars. Police said they had yet to examine whether the cylinders contained patio gas as indicated on the label.

Michael Clarke, professor of defence at King's College London, said whoever was behind the intended attack would have left a lot of evidence at the scene that would help police track them down.

He said security services would be concerned that the bomb may be part of a wider plot, and that there could be other devices planted around the capital.

"They will find out about this very quickly," he said. "Any car coming into central London would be on a lot of surveillance footage. Also nobody can make a bomb without leaving behind a lot of DNA." Prof Clarke said it was almost certain the bomb was intended to detonate today.