Detectives have been granted extra time to question a 43-year-old man arrested on suspicion of terrorism after police found guns and other weapons hidden in a car.
The man, from Kirklees, West Yorkshire, was detained in Ravensthorpe, Dewsbury, on Thursday.
He was one of seven men arrested this week after the haul was found inside the vehicle, which had been impounded for having no insurance.
Police now have until July 12 to charge or release the man, or apply for more time. Officers have already been given warrants of further detention for the six other suspects, who are all from the West Midlands.
All seven men are being held on suspicion of the commission, preparation or instigation of acts of terrorism.
A spokesman for West Midlands Police said: "The arrests followed a routine stop of a vehicle by police on the M1 motorway in South Yorkshire on Saturday (June 30).
"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.
Police said 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.
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, the force added.
The investigation follows a separate counter-terror swoop in London on Thursday, 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.