Leaflets found by British police - during an operation in which seven men were held for terrorism offences - contained threats against Prime Minister David Cameron and Queen Elizabeth, a media report said.
The seven terror suspects were held after guns, explosives and pamphlets were found in a car in West Midlands, The Sun reported.
The daily said the arms cache was "discovered by chance" after two Asian men were routinely pulled over by traffic police.
The men had no car insurance and the vehicle was impounded. It was then searched, and a shotgun, ammunition and explosives were found.
Police also found A4 size leaflets that allegedly included threats against Cameron, the queen and "any enemy of Allah".
The daily said police "stumbled on this group... they weren't looking for them".
"Counter-terrorism officers are celebrating their success. Yet the reality is they had a very lucky break," it said.
The Sun said police planned not to tell locals of the arrests, as it "could give the impression the authorities were not in control" ahead of the Olympics.