SOFIA (Reuters) - Bulgarian Prime Minister Boiko Borisov said on Monday that his government would resign if the ruling center-right GERB party candidate loses the first round of the presidential election next month.

Bulgarians vote on Nov. 6 for the largely ceremonial post, as their leaders tackle a migrant crisis, growing tensions between Russia and the West and risks from Britain's decision to leave the European Union.

On Sunday, the ruling center-right GERB party nominated parliamentary speaker Tsetska Tsacheva to stand in the election, which Borisov sees also as a confidence vote for his governance.

"At the first round, if GERB loses these elections, that same evening we resign," Borisov told reporters.

Political analysts say Tsacheva is a front-runner for the post but faces stiff competition from left-wing candidates and with the winner needing majority vote, a run-off is very likely.

Borisov did not clarify whether he would step down if the former lawyer and legal adviser did not come first for a run-off vote or if she fails to secure an outright win.

"For us, the vote is at the first round, and if we lose, that evening you will get the resignation of the whole government," said Borisov, whose minority government came to power two years ago.

Analysts say Tsacheva is a clear party nomination and her main challenge would be to win support from beyond GERB circles.

"She would need a broader support to win the run-off," said Genoveva Petrova, an analyst with independent pollster Alpha Research.

Speaking to the national BNT television Tsacheva said she would work to win the trust of all Bulgarians.

"We live in complicated times – migrant influx, terrorism threats, Brexit, heavy crises in the region. All these (risks) should unite us," she said.

The election will indicate the level of support for the center-right government ahead of general elections in 2018.

GERB is still the most popular political faction in Bulgaria, a European Union and NATO member. It is credited with stabilizing the economy and ensuring steady inflows of EU aid, recent opinion polls showed.

Pollsters, however, register increased activity among leftist voters, frustrated with rampant corruption and the slowing rise in living standards in the EU's poorest country.

(Reporting by Angel Krasimirov and Tsvetelia Tsolova; Editing by Jeremy Gaunt and Dominic Evans)