OSLO (Reuters) - Norway's opposition Conservatives have increased their lead over the ruling Labour Party, indicating Prime Minister Jens Stoltenberg's government may be on course for defeat in an election this September, a poll showed on Thursday.

The Conservatives had 33 percent support among decided voters in January, up from 30.7 percent in December while Labour's support rose to 28.5 percent from 27 percent, a poll by state broadcaster NRK showed.

A Conservative-led government could support selling down stakes in state-owned companies such as Statoil or Telenor and may open new offshore areas to the oil industry.

It is also seen reducing taxes and easing the regulatory burden for companies.

Stoltenberg's two small coalition allies are struggling to maintain the minimum four percent threshold required to get into parliament.

At the same time, the populist Progress Party, a potential ally of the Conservatives, had 18 percent support, the poll showed, raising the chance of opposition leader Erna Solberg becoming the next prime minister.

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

While Stoltenberg reshuffled his government late last year, putting his top minister in charge of health, his two consecutive terms in office work against him as Norway's electorate rarely grants governments too much time in office.

Even Gro Harlem Brundtland, considered the "mother" of Norway, spent just 6 consecutive years in office in the longest of three stints as prime minister.

(Reporting by Balazs Koranyi; editing by Jason Neely)