Four persons were arrested on Wednesday by the anti-terror police in connection with the July 7 bombings in London in which 52 people were killed.
Two men and a woman were arrested in West Yorkshire while another man was arrested in the West Midlands.
They were held on suspicion of commission, preparation, or instigation of acts of terrorism under the Terrorism Act, 2000.
All four suspects were taken to a central London police station where they will be interviewed by anti-terror officers.
The arrests of the two men -- aged 30 and 34 -- and the 29-year-old woman in West Yorkshire took place at 7 am (1230 IST). Five houses in the area were searched, including two in Dewsbury, two in Beeston - south Leeds, and one in Batley.
Two flats in Birmingham were also searched, the Metropolitan Police said. The fourth man arrested is 22 years old.
No armed officers were used in today's operation.
"This was a planned, intelligence-led operation involving the Metropolitan Police Service's Counter Terrorism Command and the counter terrorism units in West Yorkshire and the West Midlands," a police spokesman said.
"We are continuing a painstaking investigation with a substantial amount of information being analysed and investigated.
"As we have said previously, we are determined to follow the evidence wherever it takes us to identify any other person who may have been involved, in any way, in the terrorist attacks," the spokesman said.
"We need to know who else, apart from the bombers, knew what they were planning. Did anyone encourage them? Did anyone help them with money, or accommodation?" the spokesman said.
In a statement, West Yorkshire Police confirmed the details issued by the Metropolitan Police and said: "Neighbourhood Policing Team officers are meeting with local people to keep them updated and informed about activity in their areas and to reassure the wider community.
"We would like to thank people for their understanding and support at this time and would ask that it continues.
"Although we are legally limited in how much we can say, we will share as much information as we can with those living in the vicinity and with the wider community.
"As usual, local Neighbourhood Policing Teams are on patrol in the areas and we would ask anyone with concerns to speak directly to them."
Fifty-two people were killed when four suicide bombers set off devices in three packed rush hour London Underground Tube trains and a crowded bus in 2005.
The bombers -- Mohammed Sidique Khan, Shehzad Tanweer, Jermaine Lindsay and Hasib Hussain -- all died in the attacks.
Last month, three men became the first people to appear in court charged with conspiring with the four terrorists.
Mohammed Shakil, 30, Sadeer Saleem, 26 and Waheed Ali, 23, from Beeston, Leeds, were arrested in March and appeared at the Old Bailey via video link on April 20.