The police also questioned on Friday another suspected would-be suicide bomber, who the government says might be linked to Usama bin Ladin's outfit.
Police forensic experts entered two new addresses in Gloucester to the west of London and also carried out a second investigation of a nearby house where they found explosives during an initial raid on Thursday.
In London, they continued to question a 24-year-British national of Pakistani origin arrested during Thursday's raid.
Though the police refused to comment on the man's suspected intentions, newspapers reported that he may have been planning the country's first suicide attack on a target such as a sports stadium or the royal family.
Some reports said the arrested Sajid Badat had a shoe bomb, similar to the one which the jailed Richard Reid had tried to detonate on board a transatlantic airliner.
Police headquarters in Scotland Yard has announced that another man, aged 33, had been arrested by anti-terrorist officers in Birmingham in an unrelated operation.
His home and five other addresses in the central English city were being searched in what was believed to be another investigation related to international terrorism.
Home Secretary David Blunkett stressed the importance of the raids in Gloucester – part of a broader security operation that included searches in the northern city of Blackburn.
"Obviously the use of the Terrorism Act 2000 indicates the extent of the concern of the security and counter-terrorism branch of the Metropolitan police," the minister said.
Blunkett also stood by his earlier comments that "there was reason to believe that Sajid has connections with the network of al Qaida groups."
Police found what they described as "a relatively small quantity of explosives" during Thursday's raid in Gloucester but found nothing suspicious on Friday.