Umar Farouk Abdulmutallab was refused a visa
A man charged with trying to blow up a transatlantic plane had been on a UK watch-list, the government has said.
Umar Farouk Abdulmutallab, 23, was refused a visa in May and placed on a security list after he applied to study at a bogus college.
He targeted a Detroit-bound flight from Amsterdam with explosives hidden in his clothes, it is claimed.
Police are searching several London properties linked to the Nigerian ex-University College London student.
Home Secretary Alan Johnson said people on the same list as Mr Abdulmutallab could not come into the UK, although they could pass through the country in transit and were not permanently banned.
Those on a more serious watch-list for terror-related reasons would automatically have their details passed on internationally, but this is not always the case with information about a person rejected because of concerns about an academic application.
Mr Abdulmutallab had also been on the US's lowest risk watch-list since November, although it allowed him to travel on planes flying into the country.
He was placed on it by US authorities after his father contacted both Nigerian and foreign officials to express his concerns after his son had ceased contact with his family.
US FLIGHT ADVICE
Only one item of hand luggage, including items bought airside
BA and Virgin Atlantic not charging to check in extra hand luggage
Check in wrapped presents
Passengers subject to "pat-down" searches before boarding, on top of usual security checks
Customers to remain seated during final hour of flight
No access to hand luggage and a ban on leaving possessions or blankets on laps during this hour
The home secretary told BBC Radio 4's Today programme the US authorities should theoretically have been informed that Mr Abdulmutallab was on a UK watch-list, and he doubted there had been a "hiccup" in procedures.
He said the UK's border security was extremely "robust": "We have a very, very strong border in this country, we have strong security measures. We've now monitored 135 million people trying to enter the country."
Mr Johnson also said there was a need to establish the nature of Mr Abdulmutallab's activities while he studied in the UK from 2005 to 2008.
He said he did not believe that Mr Abdulmutallab had been acting alone, adding that police and security services were examining whether he had been radicalised while at University College London (UCL).
UCL has confirmed that a person of the same name and description as Mr Abdulmutallab took an engineering with business finance degree at the university between September 2005 and June 2008, when he successfully graduated.
In a statement on Monday, UCL said it was "deeply shocked" by recent events.
"During his time on the course, Mr Abdulmutallab never gave his tutors any cause for concern and was a well mannered, quietly spoken, polite and able young man," it said.
A spokesman also confirmed that Mr Abdulmutallab was president of UCL's Islamic society between 2006 and 2007.
Concerns have been raised about how Mr Abdulmutallab allegedly managed to take a device - constructed from explosives moulded around his body - onto a plane.
The home secretary said the UK intended to be "at the cutting edge" of full body scanning technology, and would "put it in place as quickly as possible".
"There is an issue of cost, and you always have to get this balance between ensuring that the security of our population, which is our primary concern, is balanced against people going about their normal daily business," he added.
Shadow home secretary Chris Grayling said: "We should not jump to any conclusions following the events in the United States, but the home secretary is being downright complacent if he thinks we have a tight and secure student visa system."
BBC security correspondent Gordon Correra said the alleged attack prompted a number of questions about Mr Abdulmutallab, as well as many concerning the relationship between authorities in the UK and US.
"The key question is what did the authorities know about him before the attack?
"In terms of the UK there has been the suggestion that he might have crossed the radar of the security service MI5 while he was in the UK, but without any indication that he was planning an attack. But was he in contact with any extremists here?"
Our correspondent also said Mr Abdulmutallab's most recent application for a UK visa raised the question of why he would want to enter the country.
Extra check-in time
Air travellers are undergoing "pat-down" searches before boarding, and being restricted to one item of hand luggage.
Several US-bound flights from London's Heathrow and Gatwick airports were delayed by two or three hours on Sunday.
US authorities introduced the measures, along with a ban on leaving seats in the hour before landing, after Mr Abdulmutallab was tackled on Northwest Airlines Flight 253 from Amsterdam as it approached Detroit on Friday.
UK airport operator BAA said airline staff were carrying out checks, and advised passengers to leave extra time to check in.