LUXEMBOURG (Reuters) - The party of Luxembourg Prime Minister Jean-Claude Juncker suffered losses in the country's elections on Sunday, with its worst showing since 1999, but it will remain by far the largest in parliament.

The center-right Christian Social People's Party (CSV), which has held the post of prime minister for all but five years since World War Two, will have 23 seats in the 60-seat parliament, down three from the last elections in 2009.

"There is a huge difference between our party and the next two following parties so I do think that we will be charged to form the next government," Juncker told reporters on the sidelines of a party conference on Sunday evening.

The largest gainer of the evening was the Democratic Party (DP), which won four seats more than in 2009, tying for second place with Juncker's current coalition partner, the Socialist Workers Party (LSAP). Both now have 13 seats.

"The conclusion of today is that there is one party of the main parties that is the big winner tonight and that is the Democratic Party," Xavier Bettel, Luxembourg City's mayor and a senior figure in the DP, told RTL Radio.

The elections were called seven months ahead of schedule after the junior coalition partner withdrew support, accusing Juncker of failing to curb abuses of power by the secret service.

As the head of the largest party, Juncker will now be given the task to form a government by Grand Duke Henri.

Philippe Poirier, politics lecturer at the University of Luxembourg, said that he believed that Juncker would chose the DP over his current coalition partner LSAP.

"The coalition has to be formed before the first week of December because there's an obligatory budget vote that month. I believe that Juncker will be prime minister with a liberal (DP) partner," Poirier said.

When asked who would be his preferred coalition partner, Juncker said it was too early to comment.

Juncker, who headed the Eurogroup of euro zone finance ministers until early 2013, is the European Union's longest-serving head of government with almost 19 years in office.

(Reporting by Michele Sinner; Writing by Robert-Jan Bartunek in Brussels; Editing by Anthony Barker)