By Nick Thorpe BBC southeast Europe reporter

The closure of the two oldest units at a nuclear power station in Bulgaria is due to begin, after many years of concern over their safety and strong pressure from the European Union.

Over the next four years, four out of six reactors at Kozloduy will be shut down, despite protests from the nuclear lobby and opposition parties that the reactors are economically necessary.

The first Soviet designed reactors at Kozloduy on the river Danube, 200 kilometres north of Sofia, came online in the late 1970s.

A series of accidents led to widespread international concerns over their safety.

Those concerns focused in particular on the growing brittleness of the reactor vessels and the lack of a containment building to cope with a major accident should one occur.

EU conditions

But Bulgarian nuclear engineers argue that repairs, carried out with international help, and supervision have resolved all safety issues.

Bulgarian engineers say safety improvements have been made

They also say that Bulgaria needs the reactors both for domestic energy supply and for export.

The country desperately wants to join the EU and was given a target date of 2007 at the Copenhagen summit a week ago.

A condition of membership is the closure of the oldest four of the six reactors at Kozloduy, which generate a little over half of the plant's total capacity.

The centre-right government, led by Simeon Saxe-Coburg, the former king, reluctantly agreed to the closures.

The first two reactors will be shut down by the end of this year - reactors three and four will be closed by 2006.

The opposition socialist party has accused the government of bowing to foreign pressure and have vowed to continue the fight.