Britain’s nuclear watchdog has agreed to allow one of the country’s oldest nuclear reactors to restart, one year after it was shut down to investigate cracks in its graphite core.

EDF Energy is expected to restart reactor 4 at its 40-year-old Hunterston B nuclear plant on the Firth of Clyde in North Ayrshire within weeks after the Office for Nuclear Regulation (ONR) said the plant was safe.

The regulator will allow the reactor to run for four months after proving that the reactor cores can still fulfil their fundamental safety requirements, despite the cracks in its graphite bricks.

Donald Urquhart, the ONR deputy chief inspector, said the regulator had made the decision to give Hunterston B’s reactor 4 the green light after a “long and detailed assessment of the safety case” submitted by EDF Energy.

He said: “Nuclear safety remains our utmost priority and we would only allow a reactor to restart with clear evidence that it remains safe for workers and the public.”

The energy company said it had spent more than £125m completing the “most extensive investigation of the reactor core that has ever been undertaken” to prove the Hunterston plant’s safety.

The reactor was shut down last March after investigators discovered more than expected cracks in the graphite core of reactor 4 and reactor 3 at the Scottish nuclear plant. Its application to restart reactor 3, which was found to have more than 350 hairline cracks in its graphite core, is still pending.

EDF Energy had expected to restart the reactors in November but was forced to delay until the end of April this year. The energy company hopes to be powering homes by the end of August.

EDF Energy said it had proved that even in the most extreme conditions, including the unlikely event of an earthquake, its reactors would continue to operate within large safety margins.

It said it would continue to monitor the reactor core and would perform frequent graphite inspections to prove the reactor met safety standards.

The French-owned energy company owns and operates all of the UK’s existing nuclear power plants, which provide about a fifth of the UK’s electricity. It is hoping to extend the reactors’ expected running lives and build new nuclear plants at the Hinkley Point C and Sizewell B nuclear sites.

The company said in 2016 it would extend the lives of its Heysham 1 and Hartlepool nuclear plants, which were due to close this year but will continue to run until 2024. The closure dates of the Heysham 2 and Torness nuclear plants will both be delayed by seven years to 2030.

EDF Energy hopes to run the Hunterston nuclear plant until 2023.

• This article was amended on 21 August 2019 to clarify the location of the Hunterston B reactor.