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The Y2K bug also affected a computer at Oak Ridge National Laboratory's Y-12 nuclear weapons plant but it did not affect operations or workers, US Energy Department officials said. 

Japan was also hit. A problem with a nuclear power plant's radiation monitoring system on Saturday was confirmed as a millennium bug. Four other computer malfunctions at nuclear plants in Japan may also have been caused by the bug. All had been cleared up yesterday. 

In the United States, the Pentagon at first withheld news of its failure. But yesterday it admitted that the problem was "significant". It reduced America's ability to monitor world events and was expected to last for several days. 

A well-rehearsed back-up system went into operation within two to three hours of the breakdown, and top priority intelligence gathering was restored. But information processing was still below normal capacity, officials admitted. The breakdown occurred just after 7pm in Washington - midnight Greenwich Mean Time. 

The problem was on the ground, in computers receiving information from part of the satellite network that takes high-definition photographs of terrestrial activities and monitors radio and telephone transmissions. John Hamre, Deputy Defence Secretary, admitted: "For a short period
of time we were not able to process the information that the satellites were sending." 

The spy satellites, in orbit about 300 miles above the Earth, are operated by the National Reconnaissance Office (NRO), and keep a close eye on such hotspots as Russia's military operations in Chechnya, North Korea's preparations for missile test launches and military activities in Iraq. 

Mr Hamre refused to go into details about the affected intelligence operations. He also refused to say whether the errant computers were at NRO headquarters in Virginia, or ground-receiving stations overseas. He said the problem did not involve America's strategic early warning defence satellites that use heat detection and other methods to watch for missile attacks. 

The Pentagon knew there were no missile malfunctions in Russia because Russian and US officers worked beside each other at a joint command centre in Colorado.  

The Pentagon had spent $3.8 billion (£2.4 billion) to upgrade its 2,000 most important systems for the year 2000. 

Almost as embarrassing as the spy network failure was the way it was concealed even as reporters were being briefed that all was well. 

"I'm happy to report that all the Department of Defence systems remain in green status," Rear-Admiral Robert Willard, head of the Y2K task force for the US Joint Chiefs of Staff, said at a briefing two and a half hours after the GMT rollover. "Our systems thus far are free of year 2000 glitches." As he spoke, programmers were frantically completing a temporary repair. 

"We did not want to release incomplete information," a senior Pentagon public affairs officer said later. 

Britain's military systems appeared to have weathered the changeover. The Ministry of Defence said the RAF had completed test flights on all its front-line aircraft to ensure on-board systems were safe. 

The international technology advisers Gartner Group said that only 10 per cent of potential bug-related problems were likely to occur in the next two weeks, with most taking much longer to surface. 

Bill Gates, the Microsoft founder, said that computer glitches would emerge in the coming weeks but they were likely to be minor. 

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