* Conservatives say want to restore stable investment climate
* Populist Progress Party in favour of tariff cut review
* Gassled case closely watched by global infrastructure funds
STOCKHOLM, Sept 6 (Reuters) - Norway's Conservative Party will review a decision by the incumbent centre-left government to cut gas pipeline tariffs if it wins elections on Monday, a party official said.
The current government plans to impose a 90 percent cut on tariffs that the subsea Gassled pipeline network charges energy companies to transport gas from North Sea production platforms to processing plants in Norway and terminals in Britain, Germany, France and Belgium.
It says the aim is to encourage exploration and aid production from mature fields.
But a group of international investors, which has put $5.1 billion into Gassled, complains that the planned cut would cost them $6.6 billion in lost earnings over the next two decades and would damage prospects for Norwegian infrastructure investment.
"We will look into this if we get into government and will consider whether we should change this decision," Conservative Party energy spokeswoman Siri Meling told Reuters.
Major investors welcomed the promise of a tariff review.
The Gassled infrastructure investors together own about 45 percent of the firm and include the Abu Dhabi Investment Authority, German insurer Allianz, Swiss bank UBS and France's Caisse des Depots.
The Norwegian state holds 46 percent via Petoro, Statoil owns 5 percent, and the rest is held by foreign energy and utilities firms.
Lead investor Solveig, which has a 25 percent Gassled stake and is owned by the Canadian Pension Fund, Abu Dhabi and Allianz, has argued strongly against higher tariffs and has hinted at withholding investment in Norway.
"We perceived the tariff cuts as a break with a long-standing tradition of predictability and stability on the Norwegian continental shelf," Solveig Chief Executive Trygve Pedersen told Reuters.
COMPETING FOR INVESTMENT
Meling, a member of parliament and possible future minister, said she had had several meetings with investors in the group and agreed with the need for a stable legal framework "because we are competing for investment with other places in the world."
The new tariffs, first announced in January, were set to become effective from October 2016, after the government postponed the measure following complaints by investors.
Opinion polls suggest the right will win Monday's election and that the Conservative Party could form a coalition government with smaller conservative parties and the anti-immigration Progress Party.
Last week, Progress Party finance spokesman Ketil Solvik-Olsen said that if his party joined a new government, it would work to restore the original tariffs.
Silex Gas Norway, which has a 6.1 percent Gassled stake and is owned by Allianz, welcomed the possibility of a review.
The opposition parties "seem to be more open to the argument about the investment climate," Silex CEO Kurt Georgsen said.
An oil ministry spokesman declined to comment on the issue so soon before the elections.
When the cuts were announced, the ministry said they were aimed at encouraging higher production in mature fields and more exploration in the frontier areas of the Arctic.
It also said that the tariffs had been set up to provide investors a 7 percent return but that the actual return was 10 percent in 2012 and was seen at 10.5 percent in 2028. The funds say the lower tariffs would cut their return to 4 percent.
Pipeline charges are based on a formula that includes capital and operating costs. Under Gassled's booking mechanism, oil and gas companies make capacity reservations, often years ahead, and pay for that capacity whether they use it or not.
The infrastructure funds bought their stakes from oil and gas companies, for whom the seven percent regulated return is too low, while the large amounts of capital they need to immobilise in the networks are a drag on their capital returns.