The discovery of the explosive devices came after one of the vehicles was set ablaze at the haulier's depot at Bruton, Somerset, early yesterday morning. A search revealed bombs secreted in the wheel arches of other lorries.
One lorry was badly burnt, and the Somerset Fire Service stated that most of the company's fleet would also have been badly damaged had the fire not been spotted by drivers who had turned up for work.
The company, AE George and Sons, is the biggest in the area and carries sheep, pigs and cattle across the country, and until recently used to do so around Europe.
Avon and Somerset police officers said one of the theories they were working on was that the attack was the work of animal welfare activists. It is believed officers from the inquiry have contacted a Scotland Yard officer who monitors the activities of such groups.
Yesterday the company refused to comment, but a spokesman for the Meat and Livestock Commission said that it "deplored attacks like this, especially where there is damage to property and danger to life. This is an attack by a small minority and it is totally senseless."
The Fire Service said both lorry drivers and firefighters could have been in danger.
About a year ago, the Animal Liberation Front claimed there was a new mood of militancy among its members. This followed the firebombings of a depot belonging to the livestock firm Spiers Haulage in Claverdon, Warwickshire, and meat lorries belonging to Quality Poultry at Brackley, Northamptonshire.
Last December, Barry Horne, an animal rights activist, was jailed for 18 years at Bristol Crown Court on arson charges. He had been charged after an operation in which a team of 50 detectives had tracked him for six weeks.
Horne was described by police as one of the ALF's most accomplished activists. Scotland Yard's Special Branch believe animal welfare militants have been busy reorganising and preparing fresh campaigns since his imprisonment.
In February, a bomb - which proved to be fake - was sent to the Roslin Institute near Edinburgh which created Dolly the cloned sheep. It was sent by an organisation calling itself the Provisional Animal Liberation Front.
In April, ALF members were said to have hijacked a demonstration at a cat breeding farm in Oxfordshire. Around 300 protesters broke through a 12ft metal fence using a wooden battering ram and stoned the owner's home.
Robert Emerson, a security consultant who has studied the modus operandi of animal welfare extremists, said: "We seem to have a steadily escalating situation.
"The activists have been claiming they are getting in a position of doing something dramatic, and this could be the beginning."Reuse content