Extra passport checks could be brought in as ministers look into how a bomb suspect was able to leave the UK, Leader of the Commons Geoff Hoon says.
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. Hussain Osman, 27 (also known as Hamdi Isaac) wanted over the Shepherd's Bush attack, arrested in Rome
Hussain Osman, also named as Hamdi Issac, is thought to have left from London Waterloo station after 21 July.
He was arrested in Rome and is facing extradition. Three more suspects are being questioned in London.
Mr Hoon said there may be extra checks at departure points from the UK. Tories say better border controls are "vital".
It is believed Mr Osman, an Ethiopian-born 27-year-old who has a British passport, left London Waterloo on a Eurostar train on 26 July, travelling to Paris and Milan before arriving in Rome.
He is suspected of trying to blow up a train near Shepherd's Bush station in west London.
Mr Osman appeared before an extradition hearing in Rome on Saturday and his lawyer suggested he would fight being returned to the UK.
Italian police are also reported to have arrested his brother, named as Fati Isaac, in Brescia, northern Italy on Sunday, news agency ANSA said.
Another brother, Remzi Isaac, was held on Friday.
Mr Hoon told the BBC ministers will be looking into claims that passport checks at Waterloo are inadequate.
He said: "I'm aware that the Home Office will be looking at that. Certainly in recent times there has been enhanced security for those leaving the country as well as for those coming into the United Kingdom.
"It's one of those arguments that we have used to support the idea of identity cards, because it is vitally important that we are able to say who is in the United Kingdom at any given time."
Hussain Osman is being linked to the Shepherd's Bush attack
The Parliamentary Intelligence and Security Committee may also investigate how the suspect avoided detection, its chairman Paul Murphy said, to "see if any lessons can be learned".
A Eurostar spokesman said a permanent passport checking point, manned by French officials, is in place at Waterloo Station.
He said that there is no permanent point manned by UK officials, but checks are implemented at times of heightened security.
The spokesman added that the Home Office and Immigration Service are responsible for passport checks, not Eurostar.
The Home Office has confirmed immigration controls were stepped up at all UK departure points - including Waterloo - after a request from police.
Officials were in place from 7 July to 17 July but were reinstated on 21 July.
But the Home Office would not comment on how Mr Osman was able to get past these checks and leave the country.
Earlier, shadow home secretary David Davis said: "This demonstrates the vital and immediate necessity for the government to get a grip on our porous borders, both in terms of people coming into the country and in terms of people leaving."
British Transport Police said they have no plans to enhance existing security at Waterloo.