TOKYO (AFP) - A novice Japanese lawmaker lambasted by critics for breaching imperial etiquette has received a death threat in an envelope that contained a knife, reports said on Wednesday.

The menacing letter, discovered by security officers at a Tokyo building filled with lawmakers' offices, warned that "a group of assassins will be dispatched shortly", public broadcaster NHK and other media reported.

Inside the envelope was a clasp knife with a 9cm blade, Jiji Press news agency said.

The threat was directed at actor-turned-politician Taro Yamamoto, who was elected to Parliament as an independent on a strongly anti-nuclear platform.

The lawmaker caused outrage by handing a letter to Emperor Akihito during a royal garden party last month, which some critics say was a breach of protocol since the revered figure is seen as being outside the political fray.

Mr Yamamoto has said the handwritten note had been his attempt to let the Emperor know directly about the plight of people affected by the atomic disaster at Fukushima.