The Japanese Prime Minister has lost the support of a coalition partner because of his decision not to move a United States airbase off Okinawa island.
The Social Democratic Party (SDP) says it's pulling out over Yukio Hatoyama's failure to remove the Futenma base.
On Friday Mr Hatoyama fired SDP chief Mizuho Fukushima from her job as consumer affairs minister because of her stance on the issue.
The SDP has few seats in parliament but the coalition deal secured the ruling party a majority in the upper house.
Poll ratings plummeting
The ruling Democrats have a large majority in the lower house, which they can use to force legislation through the upper house, but the SDP's departure will still come as a blow to Mr Hatoyama, whose approval ratings have been plummeting.
He also faces his first electoral test in July since last year's election, with polls for the upper house.
Mr Hatoyama had said during the last election campaign that he would revisit the issue with the US - but confirmed last week that the existing agreement to relocate the controversial base to the north of the island would stand.
