BISHKEK (Reuters) - Kyrgyzstan's government resigned on Wednesday after President Almazbek Atambayev's party quit the ruling majority coalition earlier this week, deepening a rift between the pro-Russian leader and his former allies.

The Social Democratic party, the biggest party in parliament, broke up with its coalition partners on Monday over their refusal to back proposed constitutional reforms.

The proposed changes would strengthen the powers of the prime minister, a role which Atambayev could in theory take after stepping down as president next year, although he said in August he had no such plan.

A bill calling a referendum on the constitutional reform on Dec.11 needs to be passed in the final, third reading to become law.

One member of the ruling coalition, the Ata Meken party, has opposed the reform and party leader Omurbek Tekebayev has become one of its most vocal critics, arguing that the change would give the prime minister too much power.

Tekebayev was, along with Atambayev, one of the leaders of protests in 2005 and 2010 that toppled two successive Kyrgyz presidents. Some other former allies, such as Roza Otunbayeva, interim president in 2010-2011, have also criticized Atambayev's reform plan.

If the confrontation between Atambayev and his opponents extends beyond a war of words and parliamentary maneuvering, it could destabilize the volatile Muslim nation of 6 million which hosts a Russian military base.

The Social Democrat and Ata Meken coalition also included the Kyrgyzstan and Onuguu-Progress parties - which supported the constitutional reform - and the bloc controlled 80 out of 120 seats in parliament. Social Democrats on their own are the biggest faction, holding 37 seats.

(Reporting by Olga Dzyubenko; Writing by Olzhas Auyezov; Editing by Jacqueline Wong, Robert Birsel)