Reuters on the resignation of deputy PM in Bulgaria

Bulgarian deputy PM quits, raising concerns for gov't stability

(Adds details, analyst quote)

SOFIA, May 10 (Reuters) - Bulgaria's deputy prime minister resigned on Tuesday after his ABV party said it would withdraw its support from the government, a move that that raises concerns over the stability of the ruling centre-right coalition.

Bulgaria, the European Union's poorest country, is on its fifth government since 2013 and rampant corruption is hampering efforts to boost economic growth and improve living standards.

Ivailo Kalfin, who was also labour minister in Prime Minister Boiko Borisov's government, stepped down shortly after ABV announced it would end its backing for the government due to disagreements over policy and changes in the election code.

It will make its final decision at a party congress on Sunday.

The centre-left ABV, led by Bulgaria's former president Georgi Parvanov, is not formally part of the two-party ruling coalition but had declared its support for it and in return had won a ministerial post.

Borisov's government, which took office in 2014 on pledges to spur economic growth, lacks an outright majority and relies on the support of ABV and the nationalist Patriotic Front to stay in power.

ABV's move will force the government to seek parliamentary support on a case-by-case basis and further sap its ability to deliver on sorely-needed economic and other reforms.

"It is obvious that the stability of the coalition is now vulnerable," said Daniel Smilov, a political analyst with the independent think-tank Center for Liberal Strategies.

"It is clear now that it will be hard for GERB (Borisov's party) to achieve its goals because every decision will depend on small parties."

Bulgaria holds a presidential election in October and some analysts say ABV wants to distance itself from the ruling coalition to increase its chances in that race. The incumbent, Rosen Plevneliev, a former GERB minister, has not said yet whether he will seek re-election to the ceremonial post.

Analysts said an early parliamentary election next spring could not be ruled out, though GERB has said this would not be good for the country.

GERB also shrugged off ABV's withdrawal of support, saying that in many cases it had not voted in line with the government.

Last December a small right-wing party withdrew its support for Borisov's government after parliament watered down changes in the constitution aimed at overhauling the corruption-riddled judiciary. (Reporting by Tsvetelia Tsolova and Angel Krasimirov; Editing by Gareth Jones)