Washington AS the shock waves from yesterday's terrorist attacks in London - which seem to be the work of jihadist militants - reverberate across the Atlantic, a grim truth should become increasingly clear: one of the greatest terrorist threats to the United States emanates not from domestic sleeper cells or, as is popularly imagined, from the graduates of Middle Eastern madrassas, but from some of the citizens of its closest ally, Britain. Richard C. Reid, the "shoe bomber" who tried to blow up an American Airlines jet flying between Paris and Miami in 2001, is British. So is Saajid Badat, who pled guilty in London four months ago to plotting to use a shoe bomb similar to Mr. Reid's to blow up a trans-Atlantic flight in late 2001. And Ahmed Omar Sheik, who orchestrated the 2002 kidnapping-murder of the Wall Street Journal reporter Daniel Pearl in Pakistan, is a British citizen of Pakistani descent who graduated from the London School of Economics. In 2004 British police arrested 12 terrorist suspects, many of them British citizens (including Qaeda operative Abu Issa al-Hindi), who were allegedly plotting attacks both in Britain and the United States. American law enforcement officials accuse Mr. Hindi of leading the surveillance of financial targets in New York and Washington between August 2000 and April 2001. Those targets included the International Monetary Fund, the World Bank, the Prudential building in Newark and the New York Stock Exchange. And two years ago, British citizens engaged in a highly unusual suicide attack outside the country, a foreshadowing of how such an attack might be mounted in the United States. On April 30, 2003, two Britons of Pakistani descent walked into Mike's Place, a jazz club near the American Embassy in Israel. Once inside, the younger of the two men succeeded in detonating a bomb, killing himself and three bystanders, while the other man fled the scene. If such an attack can happen in Israel, a country with the most rigorous counterterrorist defenses in the world, it can also happen here. Why have so many of these terrorists come from Britain? Many British Muslims are young and poorly integrated into society and therefore vulnerable to extremism. In fact, Muslims have the youngest age profile of any religious group in Britain; around a third are under the age of 16. The unemployment rate among British Muslims runs almost 10 percentage points above the national average of about 5 percent. In the case of 16- to 24-year-old Muslim men, the unemployment rate is 22 percent. Not surprisingly, polls of British Muslims show a considerable sense of anger. Eight out of 10 believe that the war on terrorism is a war on Islam, while a poll conducted last year, under the auspices of the Guardian newspaper, found a surprising 13 percent who said that further attacks by Al Qaeda or a similar organization on the United States would be justified. One rap video that surfaced in Britain last year called "Dirty Kuffar" had lyrics that included the following verse: "O.B.L. [bin Laden] pulled me like a shining star! Like the way we destroyed them two towers, ha-ha!" Last year a British government report estimated that between 10,000 and 15,000 British Muslims are supporters of Al Qaeda or related groups. The estimate was based on intelligence, opinion polls and a report that 10,000 Muslims attended a 2003 conference held by Hizb ut-Tahrir, described by the Home Office as a "structured extremist organization." British authorities believe that between 300 and 600 British citizens were trained in Qaeda and Taliban camps in Afghanistan. For this reason, and because of Britain's relatively permissive asylum laws, Arab militants living in London sometimes jokingly refer to their hometown as Londonistan. Here's the problem for the United States: Under our Visa Waiver Program, residents of Londonistan who hold a valid British passport can board a plane for the United States without an interview by an American consular official. This program also applies to more than a score of other European countries, like France, Germany, the Netherlands and Spain, that meet the criteria for visa-free travel to the United States. Unfortunately, while these countries may enjoy a low visa refusal rate, grant reciprocal visa-free travel to Americans and issue machine-readable passports - all criteria for inclusion in the waiver program - many of them have also had a hard time integrating their growing Muslim populations. Even before yesterday's attacks, there was plenty of evidence from episodes like the Madrid bombings in 2004 that these countries contained sleeper cells with the ability and motivation to carry out major terrorist operations and even, perhaps, to attack the United States itself. As declining populations in Europe are replaced in part by rising Muslim emigration from the Middle East, North Africa and South Asia, economic resentment and sectarian strife seem likely to grow. Tinkering with visa regulations might help, but it is unlikely to change the reality that Islamic militant groups in Britain, as in several other major European countries, represent a growing threat to the United States that will continue for many years to come.