(Adds SPLM, government reaction)

By Skye Wheeler

JUBA, Sudan Sept 7 (Reuters) - Campaigners said on Monday they had found serious discrepancies in reports of Sudan's oil revenues which could mean Khartoum's government was underpaying its strife-torn south by hundreds of millions of dollars.

The findings by UK-based Global Witness could spark a political storm in Sudan, where relations between its Muslim north and mostly Christian south have remained tense since the end of their two-decade civil war in 2005.

Under the 2005 peace accord, both sides agreed to share the country's oil wealth, with the south receiving half the state revenues from the oil drilled from its territory.

Global Witness said it had found revenue figures from some oilfields published by Sudan's Ministry of Finance were lower than revenue figures for the same oilfields published by operator China National Petroleum Corporation (CNPC). The Ministry figures are used to calculate the southern share.

The study "raises serious questions about whether the revenues are being shared fairly", said a statement by Global Witness, a group which campaigns against conflict and corruption related to natural resources.

Global Witness campaigner Rosie Sharpe told Reuters in an email the extent of the discrepancy varied from field to field and year to year "but is of the order of 10 percent."

The group's statement said a total undercount of 10 percent since 2005 would mean "the southern government would be owed more than $600 million".

TIMING SENSITIVE

Sharpe told Reuters the findings did not necessarily mean the Khartoum government was cheating the south out of money.

"It could be that it is the oil company that overstates the figures, although the figures do come from their annual report, an official publication of a multi-billion dollar company."

Sharpe later told Reuters at a news conference in Nairobi any long term dispute over oil revenues could reignite north-south conflict. "The lack of transparency over sharing the oil revenues fuels mistrust between the north and the south."

Sudan currently pumps some 500,000 barrels of oil a day, much of it found in the south.

Sudan's Ministry of Finance said it had passed the report on to the Ministry of Energy and Mining, which would be able to comment after it had studied the figures.

A senior member of the south's dominant Sudan People's Liberation Party (SPLM) said the report confirmed long-held suspicions about how oil wealth was shared.

"On many different occasions, we have expressed our dissatisfaction ... about the ways oil revenues are distributed. This independent report strengthens our case," said Yasir Arman.

The findings come at a sensitive time for Sudan, which has national elections scheduled in April 2010 and a referendum on southern independence in 2011. Any return to conflict would have a disastrous impact on Sudan and the surrounding region.

The Global Witness report said researchers found a 9 percent discrepancy between government and company estimates for production in 2007 from Sudan's blocks 1, 2 and 4, run by the CNPC-led Greater Nile Petroleum Operating Company.

In 2005, Global Witness said there was a 26 per cent difference between government and CNPC reports for blocks 1, 2 and 4, combined with block 6, also controlled by CNPC.

The study found a discrepancy of 14 percent for 2007 figures from blocks 3 and 7, operated by the CNPC-dominated Petrodar.

It said it had not found significant discrepancies for oilfields in north Sudan, from which Khartoum does not have to pay revenues to the south.

Global Witness said there was also a lack of transparency in how Sudan's national government deducted money from southern revenues for pipeline fees and marketing costs. (Additional reporting by Jeremy Clarke in Nairobi; Editing by Andrew Heavens)

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