The British authorities are also interested in a fifth plane, in their inquiry into the death of Litvinenko, according to Reid.
"There is one other Russian plane that we know, that we think we would be interested in. There may be other airplanes... But those are the five we know of," Reid said.
The Health Protection Agency is expected to clear one of the three British Airways aircraft identified as having possible radioactive contamination. The aeroplanes operate the London-Moscow route.
BA plane investigation
All three B767 short-haul aircraft have remained out of service for forensic testing until further notice.
"The police found very low traces of a radioactive substance on board two of the three aircraft"
Send us your views
Litvinenko, a former Russian agent who became a leading critic of Vladimir Putin, the Russian president, died of radiation poisoning in a London hospital last week.
The police found "very low traces of a radioactive substance on board two of the three aircraft" that they wished to search, the airline said in a statement on Wednesday.
BA said it has been advised that the risk to public health was low.
But it said it was contacting customers who had flown on the aeroplanes, which operate within Europe.
Litvinenko died last Thursday of radiation poisoning. Significant amounts of radioactive Polonium 210 were found in his body.