PARIS, Aug 14 (Reuters) - Belgian energy company Electrabel said its Doel 4 nuclear reactor would stay offline at least until the end of this year after major damage to its turbine, with the cause confirmed as sabotage.

On Tuesday, Electrabel had said the plant would remain offline until Sept. 15 as it carried out repairs and investigated an oil leak that forced its closure on Aug. 5. Its French parent company GDF Suez confirmed the closure was due to sabotage.

The shutdown of Doel 4's nearly 1 gigawatt (GW) of electricity generating capacity as well as closures of two other reactors (Doel 3 and Tihange 2) or months because of cracks in steel reactor casings adds up to just over 3 GW of Belgian nuclear capacity that is offline, more than half of the total.

The latest closure will put further pressure on the earnings of GDF Suez, which warned last month that the closure of the first two Belgian plants would push its 2014 group net recurring income to the lower end of its forecast range of 3.3 billion to 3.7 billion euros.

The French company said those outages would have an impact of about 40 million euros per month on net recurring income.

Electrabel said on Thursday the Doel 4 reactor had shut automatically on Aug. 5 following an oil leak in its steam turbine in the non-nuclear part of the plant. The firm said the leak had caused major damage to the turbine's high-pressure section.

"Based on this partial analysis, Doel 4 will certainly not be available before Dec. 31, 2014," Electrabel said.

Once the low-pressure parts of the turbine have been opened and examined, Electrabel will update its estimate of how long the repair work will take.

Energy experts have raised the spectre of possible blackouts this winter and say Belgium will have to boost interconnection capacity with neighbouring countries to prevent power shortages.

A GDF Suez spokesman confirmed Belgian press reports about suspicion of sabotage.

"There was an intentional manipulation," he said, adding that somebody had tampered with the system used for emptying oil from the Alstom-made turbine.

He said no outsiders had penetrated into the plant but declined to say whether an employee could have purposely caused the leak, as has been reported in some Belgian media.

He said Electrabel had filed a complaint and that the Belgian police had started an investigation.

GDF Suez shares close 0.6 percent higher on Thursday, slightly outperforming the broader market.

AGEING REACTORS

Doel 4 is the youngest of four reactors at the Doel nuclear plant, 20 km north of Antwerp, Belgium's second-biggest city. The country has three more reactors in Tihange, 25 km southwest of the city of Liege.

Doel 1 and 2, which came on line in 1975, are set to close in 2015. Tihange 1, which also started operation in 1975 and was designed to last 30 years, got a 10-year extension till 2015.

The two closed reactors Doel 3 and Tihange 2 were connected to the grid in 1982 and 1983. Doel 4 and Tihange 3, which came on line in 1985, were operating normally until the closure of Doel 4 last week.

In Britain, EDF Energy, owned by France's EDF, took three of its nuclear reactors offline for inspection on Monday after finding a defect in a reactor of a similar design.

The problems of the two French utilities with their reactors abroad may serve as a warning of possible generic flaws that could appear in EDF's ageing nuclear park at home.

With 58 reactors in 19 nuclear plants, France is the world's most nuclear-dependent country, relying on it for nearly three quarters of its power.

All of its plants are of the same basic Pressurised Water Reactor design, which means that a flaw discovered in one of EDF's reactors could force the closure of others.

French nuclear regulator ASN has repeatedly warned policy-makers that they should make room for additional supply in case a fault at one reactor prompted the precautionary closure of more than a dozen reactors. (Additional reporting by Michel Rose; Editing by Andrew Callus and Jane Baird)