Belgian power utility Electrabel said Saturday it had restarted a nuclear reactor at its ageing Tihange plant, just days after being forced to shut it down following a fire in the electricity supply system.

An Electrabel spokesman said the Tihange 1 reactor was put back on line as scheduled and would be running at full power in due course.

Last week's shutdown of the reactor was "normal procedure" after such a fire, the company said previously.

Tihange 1 is the oldest of three reactors at the plant, about an hour's drive southwest of Brussels, and began service in 1975.

It was meant to be closed completely in 2015 but the government decided in 2012 to keep it running until 2025 to help ensure electricity supplies while other energy sources are developed.

Environmentalists, especially in neighbouring Germany which is phasing out nuclear power, have criticised extending the life of Belgium's reactors which they say are too old to be considered safe.

Belgium has suffered a series of nuclear mishaps in recent years, with three of the country's seven reactors closed at one point, in two cases due to the discovery of micro-cracks in the reactor vessels.

Overnight Thursday, Electrabel was forced to halt another reactor at its Doel plant near the northern city of Antwerp, only days after it was restarted following a 21-month shutdown.

The company said at the time it expected to resume operation of the Doel 3 reactor "within a few days."

An Electrabel spokeswoman said Saturday that the Doel 1 reactor, halted since early this year and which was supposed to have been restarted on Sunday, will now come on line on Thursday instead.

Such slippage in estimated dates was "normal," she added.