Seven men were arrested on suspicion of terrorism after police found guns and other weapons hidden in a car that was impounded for having no insurance.
One man from West Yorkshire and six from the West Midlands are being questioned on suspicion of the commission, preparation or instigation of acts of terrorism following a series of arrests this week.
West Midlands officers had impounded the uninsured car, only to find two guns, some ammunition and other offensive weapons hidden inside.
– Police spokesman
"The arrests followed a routine stop of a vehicle by police on the M1 motorway in South Yorkshire on Saturday. The car was impounded on suspicion of having no insurance. Firearms, offensive weapons and other material were later found hidden inside, prompting police to take action to trace and arrest the driver, passenger and others suspected of being involved."
The car was stopped between junctions 33 and 34 of the southbound M1.
This follows a separate counter-terror swoop in London yesterday, which saw six arrests in Stratford, east London and Ealing, west London.
A former police community support officer and Muslim convert Richard Dart, 29, were said to have been among those held.
The arrests were made as part of a pre-planned operation, but the threat was not thought to be imminent.
West Midlands Police said that in the latest arrests, three men aged 23, 26 and 27 from Sparkhill, Birmingham, were held on Tuesday morning.
Three more suspects, a 22-year-old from Alum Rock, Birmingham, a 24-year-old from the Moseley area of the city, and a 22-year-old from Smethwick, West Midlands, were arrested on Wednesday evening.
A 43-year old man from Kirklees, West Yorkshire, was detained yesterday.
Detective Chief Superintendent Kenny Bell, head of the West Midlands Counter Terrorism Unit, said: "As soon as the items were discovered in the impounded vehicle, our priority was to protect the public by pursuing and arresting those we believed to be involved."
Police said the contents of the car were undergoing forensic analysis and the suspects' homes had been searched.
The guns found were not automatic weapons, West Midlands Police said.
Detectives have been given extra time to question the three men arrested on Tuesday, after the initial 48-hour detention period ran out. They now have until next Tuesday morning to charge or release the men, or apply for a further warrant.
All seven men are being held in the West Midlands.