Reid told British lawmakers that some 24 venues have been monitored for radioactive substances, and that traces had been detected at roughly half of them.
MORE: Coverage in Russian from RFE/RL's Russian Service.
Reid said, however, that there was no reason to believe public health was at risk, saying, "I would stress that the health protection agency continues to reassure members of the public that the risk of exposure to this substance remains low."
Reid's statement follows an announcement by British Airways that traces of radioactivity had been found on two of its aircraft.
With that announcement, the Litvinenko case went from a spy novel-style manhunt to a global incident with massive implications for airline security and public health:
"Early today, we had confirmation that two of our aircraft -- two Boeing 767s -- that were identified by the government have traces of radioactive material on board the aircraft," Willie Walsh, chief executive officer of British Airways, told reporters on November 29.
Walsh said three specific aircraft had been identified, all 767s. Two had been tested and found to have "very low levels of radioactive traces."
The two planes found with radioactive traces are currently at London's Heathrow Airport. A third British Airways plane, located at Moscow's Domodedovo airport, is also due to be screened. A fourth, Russian, plane, was examined and deemed radiation-free.
Authorities have refused to say whether the radioactive traces found on board the planes is polonium-210, the substance that poisoned Litvinenko.
Regardless, public exposure to the radioactivity is a factor of concern. Altogether, Walsh said, the three planes spent the month of November carrying tens of thousands of passengers between at least eight European cities.
"In total, with the three aircraft, a total of 221 flights [are involved]," Walsh said. "And we estimate that there are 33,000 passengers involved. We have identified all of the flights, all 221 flights, and they will be published on our website."
While British Airways says the risk to public health is "low," the carrier is in the process of contacting every passenger who traveled on board one of the three planes.
One of those passengers is Andrei Lugovoi, a former KGB agent who was among the people to meet with Litvinenko on the day he fell ill.
Lugovoi and a second Russian, Dmitry Kovtun, met with Litvinenko at a hotel in central London on November 1.
Lugovoi told Russia's "Kommersant" newspaper he flew from London to Moscow on one of the planes under examination on November 3. But he has denied any role in Litvinenko's poisoning.
Litvinenko, a fierce Kremlin critic who defected to Britain in 2000, was at first believed to be suffering the effects of thallium, a highly toxic metal used in rat poison.
The day after his death, however, doctors announced that what they called a "signficant amount" of polonium-210 in Litvinenko's urine.
Since then, radioactivity has been detected at a number of locations habituated by Litvinenko -- including the hotel where he met with Lugovoi and Kovtun, and a sushi bar he visited the same day.
Radioactive traces have also been found at Litvinenko's home, a security company, and the office of his close acquaintance, exiled Russian billionaire Boris Berezovsky.
Health officials have reportedly sent at least eight people to be examined for possible radiation exposure.
Polonium-210, the substance which killed Litvinenko, is highly radioactive but has a very short range. It can enter a body only by being ingested or inhaled, and then only damages tissues with which it comes in contact.
Specialists say it is "unlikely" to have contaminated anyone who came in contact with Litvinenko.
Police have labeled Litvinenko's death "suspicious" but have stopped short of calling it a murder case.
Litvinenko's friends and family, however, say the former agent was the victim of a deliberate and highly sophisticated assassination plot.
He had repeatedly accused the Kremlin and Russian special services of orchestrating terror attacks and high-profile assassinations.
These included the 1999 Moscow apartment bombings and the 2004 murder in Qatar of former acting Chechen President Zelimkhan Yandarbiyev.
The Kremlin has categorically denied any role in the Litvinenko affair, saying the plot was the work of Western forces seeking to discredit Russian President Vladimir Putin.
The case threatens to upset British-Russian relations.
British Prime Minister Tony Blair has described the incident as "very serious," and has said that "no diplomatic or political barrier" would be allowed to hamper the investigation.
Home Secretary Reid said today that Foreign Secretary Margaret Beckett had received a pledge of cooperation from her Russian counterpart, Foreign Minister Sergei Lavrov:
"The foreign secretary spoke to the Russian foreign minister on Wednesday, 29th of November, and requested all necessary assistance with the public health aspects of this incident," Reid said. "In addition, she formally requested all necessary cooperation with the ongoing investigation. The Russian foreign minister assured that this cooperation would be forthcoming."
(with agency reports)
- Polonium, also called "radium F," was discovered by Marie Curie and her husband, Pierre Curie, in 1898 and was later named after Marie's homeland of Poland (Latin: Polonia).
- It is an alpha emitter, meaning that although it is highly radioactive, it cannot penetrate human skin or a sheet of paper. Washing eliminates traces.
- Contact with a carrier's sweat or urine could lead to exposure. But polonium-210 must be ingested or inhaled to cause damage.
- Polonium-210 has a relatively short half-life of 138 days.
- Polonium-210 occurs naturally in the environment (it is found in such things as dirt and tobacco) and in people at low concentrations. But acquiring a lethal amount would require individuals with expertise and connections.
- Polonium-210 emits 5,000 times more alpha particles than radium, and an amount the size of the period at the end of this sentence would contain about 3,400 times the lethal dose. A dose like the one that killed former Russian spy Aleksandr Litvinenko would probably have been manufactured at a nuclear facility.
- Russia exports 8 grams of polonium-210 monthly, all of it to the United States. Exports to Britain ended about five years ago.