A novice Japanese lawmaker who wanted to draw attention to the Fukushima nuclear crisis has caused an uproar by doing something taboo: handing a letter to the emperor.

It began at an annual autumn Imperial Palace garden party last week. As Emperor Akihito and his wife, Michiko, greeted a line of guests, the outspoken actor-turned-lawmaker Taro Yamamoto gave the emperor the letter – a gesture considered both impolite and inappropriate.

Video of the encounter, repeatedly aired on television, shows the 79-year-old emperor calmly taking the letter, written on a folded "washi" paper with ink and brush, and briefly talking with Yamamoto. An apparently wary Empress Michiko gently pulled her husband's elbow from behind. The chief steward, who was standing next to Akihito, grabbed the letter the instant the emperor turned to him.

Yamamoto's action drew criticism from both ends of the ideological spectrum and left many Japanese baffled by what they consider to be a major breach of protocol: reaching out to the emperor in an unscripted act.

The controversy shows how the role of Japan's emperor remains a sensitive issue, nearly 70 years after Akihito's father, Emperor Hirohito, renounced his divinity following Japan's defeat in the second world war and became a symbol of the state.

Many conservatives still consider the emperor and his family divine – "the people above the clouds" – and believe a commoner should not even talk to him. Decades ago, commoners were not even allowed to look directly at the emperor, but today Akihito does meet ordinary people, including those in disaster-hit areas in northern Japan.

There is no specific law, but people are not supposed to talk freely to the emperor, touch him or hand him something without permission. Taking a mobile phone picture of the emperor or his family is also considered impolite.

An upper house committee is discussing whether to discipline Yamamoto and a decision is expected this week. The 38-year-old lawmaker, who was elected in July as an independent, has apologised for troubling the emperor but rejected calls to step down.

Yamamoto, an anti-nuclear activist, said he wanted to make an appeal to the emperor about the crisis in Fukushima and its possible health impact on residents and workers cleaning up the power plant, which suffered three meltdowns after it was devastated by the 2011 earthquake and tsunami.

Neither Yamamoto nor the palace has released the letter's contents. If Yamamoto sought the emperor's assistance, he may have violated a law requiring cabinet approval for such requests.

Yamamoto denied any intention to use the emperor for political purposes – a possible infringement of the postwar constitution, which relegates the emperor to a non-political, ceremonial role.

"My behaviour was indiscreet for a place like the garden party," Yamamoto said at a news conference on Tuesday. "I just wanted the emperor to know the reality. I was frustrated by not being able to achieve any of my campaign promises yet."

Liberals criticise Yamamoto for turning to the emperor for help rather than upholding democratic principles as an elected lawmaker. Some worry that Yamamoto's ploy reinforced the idea that the emperor is Japan's most trusted public figure, and fear that could play into conservative efforts to give the emperor more powers.

Others criticise Yamamoto as an amateur and populist politician who has set back the anti-nuclear movement, said Koichi Nakano, a political scientist at Sophia University in Tokyo.