Six men and a woman have been arrested in Birmingham in a large anti-terrorism operation, West Midlands Police say. The men, aged between 25 and 32, are being held on suspicion of the commission, preparation or instigation of an act of terrorism in the UK. The woman, 22, is held on suspicion of failing to disclose information contrary to the Terrorism Act 2000. The arrests are thought to be the most significant this year, BBC home affairs correspondent Danny Shaw said. It is understood the investigation, which has involved security service MI5, relates to suspected Islamist extremism. \'Ensure safety\' The men were taken into custody by unarmed officers in the Moseley, Sparkbrook, Sparkhill, Ward End and Balsall Heath areas of Birmingham between 11:30 BST on Sunday and 1:00 BST on Monday, police said. Police said specialist teams were searching their homes and seven other properties in the city - six domestic and one commercial. West Midlands Police Assistant Chief Constable for security Marcus Beale said: \"The operation is in its early stages so we are unable to go into detail at this time about the nature of the suspected offences. \"However, I believe it was necessary to take action at this time in order to ensure public safety.\" The terrorism threat level in England, Wales and Scotland remains unchanged at \"substantial\" - meaning that an attack is a \"strong possibility\". Our correspondent said the fact the arrests were carried out by unarmed officers suggested there was no imminent threat suspected.