Police investigating the failed bomb attacks in London on 21 July have arrested six men and a woman after two raids in Brighton.
The seven held at this Brighton flat are being questioned by police
Scotland Yard said no armed officers were involved in the Sussex raids.
The arrests bring to 19 the number of people held in connection with the attempted attacks, including the four suspected would-be bombers.
Commons leader Geoff Hoon says ministers will investigate how one was able to leave the UK after the attacks.
Ethiopian-born Hussain Osman, 27, who has a British passport, is believed to have left London Waterloo on a Eurostar train on 26 July, travelling to Paris and Milan before arriving in Rome.
SUSPECTED BOMBERS ARRESTED
1: Yasin Hassan Omar, 24, wanted over bomb attempt on a Tube near Warren Street, arrested in Birmingham
2: Muktar Said Ibrahim, 27, suspected of attempting to bomb a No 26 bus in Shoreditch, arrested in North Kensington, London
3: Ramzi Mohammed, wanted over failed attempt to bomb a Tube near Oval, arrested in North Kensington, London
4. Osman Hussain, 27 (also known as Hamdi Issac) wanted over the Shepherd's Bush attack, arrested in Rome
He is suspected of trying to blow up a train near Shepherd's Bush station in west London.
Speaking on the BBC, Mr Hoon said extra passport checks are being considered at departure points from the UK. The Tories said immediate action on border controls was "vital".
A Eurostar spokesman said that there is no permanent point manned by UK officials, but checks are implemented at times of heightened security.
The Home Office said immigration officials were in place at all British departure points from 7 July to 17 July. They were reinstated from 21 July.
Officers from Sussex police and Scotland Yard raided the two residential addresses at 0830 BST on Sunday.
All of those arrested were detained at one property - a first floor flat in a building known as Fairways in the west of the city.
They are being held under the Terrorism Act on suspicion of the commission, preparation or instigation of acts of terrorism, or harbouring fugitives. They are being questioned at a Brighton police station.
Searches are continuing at both the raided addresses. Police said there was no danger to the public in the surrounding area.
Since the arrests on Friday of those suspected of carrying out the failed bombings, police are focusing on finding those who helped plan and finance the attacks, as well as those of 7 July.
But a police spokeswoman earlier played down claims that officers were hunting a third cell poised to strike.
"We are searching for other people in connection with this ongoing inquiry. We have never spoken about a third cell.
"There were quite a few other people involved in the incidents of the 7th and the 21st. It's extremely likely there will be other people involved in harbouring, financing and making the devices."
Police have also been allowed extra time to question Yasin Hassan Omar, the man suspected to trying to blow up a train between Oxford Circus and Warren Street.
He can be questioned until Wednesday. He was arrested in Birmingham on 27 July.
Suspects can be held for a maximum of 14 days under the anti-terror laws, before they must be charged or released.
A man arrested on Sunday under the Terrorism Act at Stockport train station has been released without charge.
A spokesman said the arrest was not connected with any other inquiry at this stage.
In other recent developments:
- Italian police arrest a brother of Hussain Osman, named as Fati Isaac, in Brescia, in northern Italy on Sunday, news agency ANSA said. Another brother, Remzi Isaac, was held on Friday
- Mr Osman appeared before an extradition hearing in Rome on Saturday. His court-appointed lawyer Antonietta Sonnessa suggested he would fight being returned to the UK
- Yasin Hassan Omar, Muktar Said Ibrahim and Ramzi Mohammed, the other men suspected of carrying out the attempted attacks on 21 July, are being questioned in London
- A fifth suspect named as a brother of Ramzi Mohammed, also arrested during Friday's raids, is also being questioned. Police forensic teams are still searching the two addresses raided
- Teams are also studying items seized from other addresses in London and Birmingham