The outcome was taken by British counterterrorism experts as a triumph for the elaborate electronic surveillance techniques deployed by the country’s web of intelligence and security agencies. But the case also served as a warning of potential threats as London prepares a security force of nearly 25,000 men and women, including troops, to protect the 2012 summer Olympic Games, which begin in July.

The nine had been set to plead innocent to terrorism charges but changed their pleas to guilty at the last minute when the judge warned them of heavy sentences, including up to 13 and a half years for the man suspected of being the ringleader, if the case went to a prolonged trial. By pleading guilty, legal experts said, the men made themselves eligible for a “discount” of as much as 10 percent in their prison terms when sentenced next week.

For years, Britain’s top counterterrorism officials have been warning of what they called a network of Islamic terrorist cells across the country, and saying that the security agencies, despite major increases in financing and staff, lack the resources to track them all. Bob Quick, a former head of counterterrorism at Scotland Yard, said in a BBC radio interview that the plot “serves to remind us that there are still people out in the country contemplating and capable of carrying out terrorist attacks on innocent people.”

But other experts said that the arrests and convictions of the men showed the vast improvements in British methods of hunting down would-be terrorists.

Alexander Carlile, a member of the House of Lords and formerly the government’s independent reviewer of counterterrorism legislation, said the tracking of the militants in the stock exchange plot had been “an excellent example of the surveillance and interception capabilities of British intelligence, as good as and probably better than any other country in the world.”