Six British men have pleaded guilty to a potentially deadly bomb plot that targeted a public rally of a prominent right-wing group.

One of the men was under police surveillance, but the plot only came to light after the would-be terrorists' car was stopped in a random traffic check.

In June last year, the English Defence League was holding one of its regular rallies, railing against the dangers it says are posed by Islam.

Five men from the West Midlands packed the boot of their car with a small arsenal of weapons: a homemade bomb containing hundreds of nails and ballbearings, sawn-off shotguns, knives and swords.

The men and a sixth accomplice had also prepared a long letter outlining their motivation.

The document referred to the Queen as a "female devil" who was fooling a nation of "blind sheep".

The English Defence League (EDL) was referred to as the "English Drunkards League".

"O enemies of Allah," the letter said, "we have heard and seen you openly insulting the final Messenger of Allah."

The penalty for this, the letter said, was death.

Marcus Beale, assistant chief constable of West Midlands Police, said the men's capability was clear.

"They'd created devices that would have certainly maimed and would have possibly killed people, depending on how close they were," he said.

"In my view, they were very dangerous."

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By the time the men got to the rally though, it had finished.

They had some fish and chips, got back in their car, and began to make their way home along the M1 motorway.

A police officer who carried out a random traffic check impounded the car after discovering it was not insured.

The weapons cache in the boot was only discovered when the car was searched days later.

A manhunt soon led to the arrest of the six men.

One of them was already known to the police. He was an associate of a group of men jailed last week for planning an attack to rival September 11.

He had been observed by police just a few days before the planned attack emerging from a shop with a bag, later known to have contained the knives found in the car.

"The information that we had at the time didn't require or didn't determine that we should have been doing closer surveillance on him," Assistant Chief Constable Beale said.

"I'm really, really comfortable that within both my own organisation and in partner organisations we didn't fail to join the dots."

The case has led some commentators to call for police to be given greater powers to access data on internet servers on suspected terrorists.

"We shouldn't listen to the people who say this is an interference with our civil liberties," said Anthony Glees, head of the Buckingham University Centre for Security and Intelligence.

"The basic civil liberty is not to be bombed."

The six men will be sentenced in June.