Experts investigating the death of former Russian spy Alexander Litvinenko have found radiation traces at 12 locations, British Home Secretary John Reid said on Thursday.

Reid revealed that 24 locations including two British Airways planes have been monitored, Sky news reported.

Litvinenko, an ex-KGB man who was a strong critic of Russian president Vladimir Putin, died last week of radiation poisoning.

Traces of the same radiation substance were found on two BA planes on Wednesday. A third is being held in Moscow until it is safe to return.

Reid said examinations had been carried out on a fourth plane which is leased by Transaero and flew into Heathrow from Moscow on Thursday morning, but was later cleared.

There have been concerns about a fifth plane, also a Russian aircraft, he added.

BA is contacting some 33,000 passengers and 3,000 staff since announcing on Wednesday night that "low levels of radioactive traces" had been found on two of its aircraft at Heathrow Airport in London.

A BA spokesman said the planes which had been flown between London and Moscow were being examined because "individuals involved in the Litvinenko case" had traveled on them.

The alert involves 221 flights within Europe made by the three short haul 767s.

In a statement made to the public, the British Airways said it has been advised that "this investigation is confined solely to these three B767 aircraft, which will remain out of service until further notice."

About 5,500 passengers have rung a BA helpline since the radiation alert was issued on Thursday night, whereas some 1,700 calls have been made to the National Health Service following the radiation scare.

Some 69 people have been referred to the Health Protection Agency as a precaution. Of the 29 tested, none had worrying results. Another 18 had been referred to specialist clinics, according to Sky news.

(Xinhua News Agency December 1, 2006)