OSLO (Reuters) - Norway's centre-right opposition widened an already big lead over the ruling Labor Party in opinion polls, putting it firmly on course to win elections in less than three months, a fresh poll showed on Thursday.

Prime Minister Jens Stoltenberg, in office since 2005, has presided over a rare economic success story thanks to Norway's immense oil wealth, but voter discontent has increased over a lack of improvement in health, education and social services, as well as over a number of high-profile sex scandals.

Even the economic success appears increasingly fragile.

Growth on the mainland, a respectable 3.4 percent last year, will slow to 2.4 percent this year, Statistics Norway predicted earlier, while unemployment has reached 100,000, above levels seen at the height of the recent global downturn.

In the latest poll, the Conservatives had 32 percent support among decided voters in June, up from 31 percent a month ago, while the populist Progress Party, their potential ally in a coalition government, saw its support rise to 19.9 percent from 16.2, a poll in daily Dagsavisen showed.

Support for Stoltenberg's Labor fell to 25.2 percent from 28.7 percent with its two small allies hovering around the four percent threshold needed to get into parliament in the September 9 election.

The Conservatives, led by Erna Solberg, promise to ease government regulation, sell down stakes in state firms, reduce taxes and make government more business-friendly.

(Reporting by Balazs Koranyi)