Africa's most populous country, which relies on fuel imports because it lacks the capacity to refine its own crude oil, tried to remove fuel subsidies last year but was forced to partially reinstate them after a wave of strikes and protests.

A parliamentary investigation later found the subsidy's administration had facilitated around $6 billion of corruption over three years, with half the approved fuel imports never arriving or being sold to neighbouring countries.

"Nigerian authorities have taken some measures to improve controls," said Marc Gueniat of Swiss NGO The Berne Declaration which campaigns against corruption in the developing world.

"But the fact that certain companies accused of participating in the fraud are continuing to benefit from allocations raises the question of whether the political will to change is sincere," he added.

Nigeria's gasoline subsidy soaked up 1 trillion naira ($6.2 billion) last year, equivalent to 20 percent of the federal budget and exceeding a budgeted 888 billion naira.

The list of gasoline importers compiled by Reuters using information from five sources showed around 3.4 million tonnes was allocated for the third quarter to more than 40 companies, expanded from 30 last year.

The list showed four companies that failed to cooperate with parliament's probe were named as suppliers. These were Nepal, Fresh Synergy, Ibafon and Techno, which the parliamentary report showed collectively claimed for subsidies of nearly $60 million.

Nepal's website lists its CEO as barrister Ngozi Ekeoma who has twice been arraigned by Nigeria's Economic and Financial Crimes Commission in relation to fraudulent subsidy payments. She has pleaded not guilty to the charges. Phone calls to her company went unanswered.

At least three other companies awarded third quarter allocations - Masters, Matrix and MRS - were also ordered to account for their shipments or refund falsely claimed subsidy money in another government report released last June.

Matrix provided documents to Reuters showing it had since been exonerated by Nigerian authorities.

The other firms declined or did not respond to repeated requests for comment by email and telephone. It was not clear if these companies had since repaid their debts or been cleared.

The finance ministry would not comment on specific firms but provided a document which showed that in total only 14 billion naira out of 232 billion in questionable claims, or around 6 percent, had so far been refunded.

The ministry has previously said importers will not be delisted so long as they paid back money owed to the state.

EFFECTIVE CONTROLS?

Absent from the list are large trading houses like Vitol , Mercuria and Trafigura which have historically played an important role in supplying fuels to Nigeria but have been replaced by local firms.

However, some still provide fuels indirectly to the country or via crude-for-product swap deals.

The government has arraigned some fuel marketers and industry sources say fuel regulator the Petroleum Products Pricing Regulatory Agency (PPPRA) has introduced measures designed to limit abuse of the system.

PPPRA did not respond to request for comment.

Nigeria has already spent $1.2 billion on subsidy payments this year and economists say any sign of a spike in subsidy costs could risk Africa's second largest debt issuer spending oil savings or widening its budget deficit.

Industry sources suggested the rise in the number of importers may have been partly an attempt to stave off future supply problems, as some importers struggled to get bank loans.

"What they are doing now is giving smaller allocations to more and more companies because of the credit situation. The number is creeping higher and it could be a cause for concern if it continues," said Dolapo Oni, oil and gas analyst at Ecobank.

The supply list showed Nigerian energy firm Oando PLC (>> Oando Plc) won the biggest allocation of 135,000 tonnes while Total (>> TOTAL) and Folawiyo, in which global commodity merchant Glencore (>> Glencore Xstrata PLC) is a minority stakeholder, won 90,000 tonnes each.

($1 = 161.2000 Nigerian naira)

(Additional reporting by Dmitry Zhdannikov in London; editing by Tim Cocks and James Jukwey)

By Emma Farge and Joe Brock

Stocks treated in this article : TOTAL, Glencore Xstrata PLC, Oando Plc