UPDATE 1-British Energy says reactors to reopen by Dec. 31

(Updates throughout)

LONDON, Sept 5 (Reuters) - British Energy BGY.L got regulatory approval on Aug. 21 for boiler repairs done at its Hartlepool and Heysham 1 nuclear power stations and plans to restart them before Dec. 31, a spokeswoman for the company said.

"We have had the approval for the replacement programme," she said, adding that the four reactors would be started one by one between Oct. 1 and Dec. 31.

"We will have the phased return to service of all four units. That is still very much the plan ... Everything is on course."

The two power plants have been shut down since last autumn for repairs to their boiler closure units (BCU).

The work programme had to be checked and approved by the Nuclear Installations Inspectorate (NII) before any of the units could be restarted.

Work at the plants is ongoing but getting NII approval for the work should mean no lengthy waiting for the all clear to restart when the work is complete.

The nuclear regulator is working with British Energy engineers at the plant.

British Energy operates eight nuclear power stations built since 1965 and in recent years it has struggled with issues linked to ageing.

(Reporting by Daniel Fineren)