HANOVER, Germany (Reuters) - The German state of Lower Saxony will hold new elections on Oct. 15, after a Greens party member defected to the conservatives on Friday, costing the current Social Democrat-Greens coalition government its one-seat majority, Premier Stephan Weil said.

Weil, a Social Democrat (SPD), told reporters on Monday that was the earliest possible date for state elections and was agreed by the political parties represented in the state legislature in consultation with the state election director.

Bernd Busemann, president of the state legislature, said the earlier date would not have given enough time to send out ballots by mail and could have harmed the chances of smaller parties, who lack the infrastructure of larger parties.

Weil and Chancellor Angela Merkel’s conservatives had initially hoped to hold the state election on Sept. 24, the same day as national elections, when polls show Merkel is likely to win a fourth term in office.

Conservatives have called for Weil's resignation after a news report said he had softened an October 2015 speech about Volkswagen AG's actions in the diesel emissions scandal VOWG_p.DE at the company's request.

Weil denied the report and said the controversy was clearly politically motivated.

He did not address the issue in Monday’s brief news conference.

Hubertus Heil, secretary general of the SPD, said the party continued to support Weil, and said the allegations had been proven to be unfounded.

The defection of Elke Twesten to Merkel’s Christian Democrats (CDU) marks the latest in a series of setbacks for the center-left SPD, which is trailing the conservatives by a double-digit score in national polls.

The SPD has already ceded power to the conservatives twice this year in state elections in Schleswig-Holstein and North Rhine-Westphalia. The party is also trailing the CDU in Lower Saxony, a rich agricultural state that also hosts major companies including Volkswagen.

Twesten’s unexpected party switch came five months before scheduled elections in the state, leaving the ruling alliance with 68 seats in the 137-seat assembly.

Following this year’s regional elections and the Lower Saxony defection, the SPD now heads viable administrations in only two of Germany’s 16 federal states: Bremen and Hamburg.