The Russian Orthodox Church will choose tomorrow between three alleged former KGB agents as its next spiritual leader.

More than 700 priests, monks and lay representatives will decide who should become the new Patriarch in the first Church election since the collapse of the Soviet Union.

The contest at Christ the Saviour Cathedral in Moscow pits the favourite, Metropolitan Kirill of Smolensk and Kaliningrad, against two rivals who also rose through the heirarchy at a time when the Church was under strict Communist control.

Metropolitan Kliment of Kaluga and Borovsk, 59, is in charge of economic affairs, and Metropolitan Filaret of Minsk and Slutsk, 73, is head of the Church in Belarus. The winner of the contest, which could last until Thursday, will be the 16th Patriarch of a Church with 165 million believers.

The election is taking place following the death of Patriarch Aleksiy II in December. He was elected in 1990 as the Soviet regime neared collapse and oversaw a resurgence in Orthodox belief in Russia, opening thousands of new churches.

The Patriarch's reputation was tainted by allegations that he had been a long-serving KGB agent codenamed "Drozdov" (thrush), who had been awarded a "certificate of honour" for his service in 1988.

Material from the KGB archives examined by a parliamentary committee led by a dissident priest, Father Gleb Yakunin, in 1992 also revealed that most of the Church heirarchy was infiltrated by the secret police.

Kirill, 62, was alleged to be an agent codenamed Mikhailov and Filaret was identified as agent Ostrovskii. Kliment has been accused of working as a KGB agent named Topaz, although the documentary evidence is more sketchy.

Metropolitan Filaret, who has held his post in Minsk since 1978, was head of the Church's external relations department in the 1980s. Metropolitan Kirill has been head of the same powerful department since 1989.

Kliment, who completed his studies in 1974, made official visits to the United States and Canada in the 1980s. Antoine Niviere, editor of the Orthodox Press Service in Paris, described him as "a man of the shadows of the system".

A former KGB officer named Shushpanov broke cover in 1992 to reveal that most of those who worked in the external relations department were agents, expected to report on contacts with foreigners at home and abroad.

Felix Corley of Forum 18, a body that monitors religious freedom, said that there was no doubt that senior Church leaders collaborated with the KGB.

Mr Corley, an academic who studied the archive materials, said: "It's quite clear that you could not be named a leader without being a signed-up KGB agent. They would not allow anyone to go abroad and represent religious organisations without it being controlled by the KGB."

Kirill won the backing of half of senior bishops at a meeting on Sunday to draw up a shortlist of candidates. He is seen as a moderniser willing to foster better relations with the Vatican.

He is also a strident voice of Russian nationalism in relations with the West, a factor that appeals to the Kremlin. However, he is also seen as determined to emphasise the Church's independence from the state as a force in society.

Some observers say that this could make him an uncomfortable choice for the Kremlin, which may prefer Metropolitan Kliment. He is seen as the standard-bearer of traditionalists and more willing to be subservient to the Kremlin.

Filaret is close to Belarus' dictatorial president Alexander Lukashenko and seen as a safe option if divisions become too intense between supporters of the other two candidates.

The Orthodox Church grew increasingly powerful under Vladimir Putin, the former KGB officer who openly professed his faith as president and now prime minister.

The late Patriarch was seen regularly beside Mr Putin and blessed Dmitri Medvedev on national television on the day of his inauguration as president, even though Russia is ostensibly a secular state.