Japanese Prime Minister Yasuhiro Nakasone's prospects for an extension in office this fall suffered a new blow today when parties in the Japanese parliament agreed to measures that will hamper his ability to call snap elections.

Conducting simultaneous elections this summer for the two houses of the parliament, or Diet, and doing well in them have been considered crucial if he is to break with tradition and hold office for a fifth year.

Some Japanese newspapers and politicians today put the lame duck label on Nakasone, who has been Japan's most enduring prime minister in more than a decade. His term ends Oct. 30.

Nakasone never has confirmed that he wants an extension of his term, but his statements have left the door open.

Analysts conceded Nakasone might still surprise his rivals and somehow engineer elections and an extension. But new attention is being focused on the three men in line to replace him: Finance Minister Noboru Takeshita, Foreign Minister Shintaro Abe and the ruling Liberal Democratic Party's executive council chairman, Kiichi Miyazawa.

The parties' decision came three days after the Tokyo economic summit, which damaged Nakasone's prestige in political circles because he failed to secure measures to stop a six-month climb in the value of the Japanese yen, which is harming the Japanese economy by hampering exports.

Last year, the Japanese Supreme Court ruled that a Diet election in 1983 had been unconstitutional and put Nakasone under pressure to reduce an imbalance between rural and urban representation.

Under a plan approved by the ruling and main opposition parties, one seat would be added to eight of the most populous districts and one seat taken away from seven sparsely populated ones.

The key element for Nakasone is a 30-day "notification period" during which he would be barred from dissolving the lower house, and therefore not be able to hold simultaneous elections in both houses. Such elections usually bring a big turnout favorable to the ruling party.