The new reactor at the Flamanville nuclear power plant in France can go online 12 years later than originally planned.
France's nuclear supervisory authority ASN authorized the start of operation of the reactor on the shores of Channel on Tuesday. This means that the operating electricity company EDF can start loading the plant with fuel.
EDF plans to produce electricity in the reactor for the first time in the summer.
Construction of the controversial reactor began in 2007 and was originally scheduled for completion in 2012.
The latest estimated costs of €13.2 billion ($14.2 billion) are four times higher than originally planned. Leaking weld seams in the steel shell were one of the problems that caused delays.
France relies heavily on nuclear energy for its power supply.
President Emmanuel Macron wants to significantly expand the country's nuclear sector. The government is considering the construction of eight new power plants by 2050 and also wants to extend the operating times of more than half of the 56 nuclear power plants in France if safety permits.