TOKYO (Reuters) - The operator of the wrecked Fukushima nuclear plant, Tokyo Electric Power Co, said it would not be able to meet a self-imposed deadline to decontaminate water containing highly radioactive substances by the end of March.

The admission by the utility known as Tepco is another setback in its struggle to cope with the contaminated water, which is mostly contained in hastily constructed tanks.

Tepco President Naomi Hirose visited officials at the Nuclear Regulation Authority on Friday to report that the company would not be able to process all the contaminated water by the end of March as promised.

"I feel very sorry that I was not able to carry out my promise to process (the contaminated water)," Hirose said.

"At our current pace we will complete it by mid May," he added.

Tepco has been fighting a daily battle against contaminated water since Fukushima was wrecked by an earthquake and tsunami in March 2011 and three reactors underwent meltdowns.

Water flushed over the wrecked reactors to keep them cool enough to prevent further radioactive releases is leaking into basements after contact with the melted uranium fuel.

The water mixes with groundwater that seeps into the basements, requiring more pumping, treatment and storage.

Missteps and leaks have dogged the efforts to contain the water, slowing down the overall decommissioning process, which is expected to take decades.

(Reporting by Kentaro Hamada; Writing by Aaron Sheldrick; Editing by Alan Raybould)