LONDON – Around the world, watch lists are a key tool against terrorism – but highly imperfect.

Experts say simple issues such as fickle spelling and incomplete data, as well as deliberate deception and uncooperative countries, all make it possible for a determined terrorist such as bomb suspect Umar Farouk Abdulmutallab to slip across borders.

British officials are proud of their list, which contains more than 1 million names, including that of Abdulmutallab. That didn’t stop the young Nigerian from boarding a flight from Amsterdam to the United States with explosives in his underwear – a stark reminder of the perils of flawed information-sharing and the limits of watch lists.

“Lists are valuable in making sure governments around the world are able to track individuals,” said John Harrison, an aviation security specialist at Singapore’s S. Rajaratnam School of International Studies. “But you don’t want to put too much emphasis on these lists. It’s an overstatement to say: ‘OK, now we are safe.’ ”

Analysts say human intelligence, information-sharing and data analysis are also vital to stopping terrorists, and Britain has announced an urgent review of its watch list system in the wake of the Christmas Day attack over Detroit.

The British list holds the names of everyone from suspected terrorists and radical clerics to wanted criminals and rejected visa applicants – such as Abdulmutallab, who was added after being denied a student visa in May 2009 for listing the name of a bogus college on the application.

The list is the centerpiece of a program called E-borders, which will eventually check all passengers traveling to or through Britain against the master list. Information comes from police, intelligence services and other sources and is held by the U.K. Border Agency.

The list has its limits, though. Names on it are not automatically shared with other countries, although those on a smaller terrorism-related watch list are.

U.S. authorities have said Abdulmutallab was in a database of 500,000 people suspected of terrorist ties but was not on a no-fly watch list. Britain has said it had no indication the Nigerian was planning an attack and did not flag him to U.S. officials as a particular threat.

On Thursday, President Obama announced about a dozen changes designed to fix the system that let Abdulmutallab slip through, including an overhaul of the nation’s terrorist watch lists.

Alain Chouet, former chief of the security intelligence service at France’s counterintelligence agency, estimated that lists he saw when working in intelligence were only about “10 percent viable.”

“The identity of people outside the European tradition is vague. People can change their names, and there is the problem of transcription into European alphabets,” he said.

The Home Office said biometric data, which are being introduced on passports and required from all visa holders, would help tighten up the system.

Experts agree that biometrics, which includes physical traits such as face recognition, are key to ensuring names on a watch list can be matched to a real individual.

Around the world there are multiple – and often overlapping – watch lists. In addition to national lists, Interpol established a Terrorism Watch List in April 2002 in response to the Sept. 11, 2001, attacks. The list can be viewed by personnel of the police agency and by authorized police forces in about 180 countries.

It scored a success in November when authorities in Indonesia were tipped off about Abdul Basir Latip, a suspected al-Qaida-linked militant from the Philippines. Acting on a tip from Interpol, which was sharing information from a U.S. watch list, officials nabbed him Nov. 21 at Jakarta’s international airport.