A Diet member who handed a letter to Emperor Akihito at a garden party last week said Tuesday he won’t step down despite harsh criticism that his act was a grievous breach of protocol.

Taro Yamamoto, an independent in the Upper House, did offer a public apology.

“I regret that what I did has troubled the Emperor,” he said at a news conference.

He also said he was not sufficiently aware of the consequences of his action.

Yamamoto, known for his anti-nuclear activities, handed the letter to the Emperor last Thursday. It reportedly detailed the current working environment at the shattered Fukushima No. 1 nuclear plant.

Yamamoto has denied using the Emperor for political purposes.

The Constitution defines the Emperor as the “symbol” of Japan and stipulates “he shall not have powers related to government.”

On Tuesday, Yamamoto told Mitsuhide Iwaki, chairman of the Upper House Standing Committee on Rules and Administration, that he intends to stay on as a lawmaker. The committee is discussing whether to discipline him.

The Imperial Household Agency criticized Yamamoto on Tuesday, saying his action was “inappropriate.”

“It was a matter to be decided in line with common sense,” Shinichiro Yamamoto, vice grand steward at the agency and no relation to the lawmaker, said at a news conference. The garden party is an occasion “for thanking people from various sectors for their services.”

The letter was not read by the Emperor and has been retained by a member of the agency, Yamamoto said.