"We decided to leave the government at an executive meeting," Fukushima told a televised news conference.

"Everybody told me it was good that I have stuck to my beliefs," she added.

The SDP has only minimal representation in parliament whose lower house is dominated by Hatoyama's Democratic Party.

The SDP's departure could further damage Hatoyama's standing as his approval ratings hover at all-time lows of around 20 per cent.