EDF Energy said on Thursday that four of its nuclear reactors, closed in northern England last month after the discovery of a crack, should resume operations by the end of the year.

The group, the British unit of French giant EDF, had decided on August 11 to shut down two reactors at each of the Heysham and Hartlepool nuclear plants as a precaution after a "defect" at Heysham 1 Reactor 1 was found during regular maintenance in June.

EDF Energy took the precautionary move to shut Heysham 1 Reactor 2 and Hartlepool Reactors 1 and 2, owing to their similar design.

The company added however on Thursday that the defect at Heysham 1 Reactor 1 was a "crack" on a component known as a boiler spine.

EDF Energy added that its engineers had completed two initial inspections and found no other defects on the boiler spines.

The company will continue with its detailed boiler inspection programme at both nuclear plants, as it seeks to understand the cause of the crack, and implement modifications as necessary, it said.

"Now that the programme has been developed in detail and is underway, EDF Energy can give information on the estimated dates for returning the four reactors to service," the group added.

"Depending on the progress of the programme and any necessary modifications, the company expects there to be a phased return to service between the end of October and the end of December, 2014."

The group had previously stated last month that the shutdown would last for at least eight weeks.

An EDF Energy spokesman told AFP that it did not have any estimate of the financial impact so far.

In a separate statement, British energy group Centrica revised the impact on earnings this year, due to its 20-percent interest in EDF Energy's existing nuclear operations.

"On the basis of the latest estimates for returning the reactors to service, and the associated costs, Centrica's earnings per share in 2014 are now expected to reduce by between 0.6 and 0.9 pence per share," Centrica said in a statement.

The London-listed group had estimated last month that the shutdown would reduce earnings per share by around 0.3 pence per share.

Following Thursday's announcement, Centrica's share price fell 0.43 percent to 323.8 pence in London. EDF Energy parent EDF was also down 0.44 percent at 24.9 euros in Paris deals.

The news comes as Britain's government is placing nuclear power at the heart of its low-carbon energy policy, in stark contrast to Europe's biggest economy Germany, which vowed to phase it out in the wake of Japan's 2011 Fukushima disaster.

Last year, Britain signed a 16-billion pound deal with EDF to build two reactors at Hinkley Point C, southwestern England, to help meet the country's future energy needs.