Iraqi money cannot be accounted for by occupying forces responsible for the funds, according to two new reports.
Iraq has some of the world's biggest proven oil reserves
Discrepancies are highlighted in the handling of $20bn (£11bn) generated from Iraq's oil and other sources since war ended last year.
The Coalition Provisional Authority was given responsibility for the country's finances by the United Nations.
The UN stressed that money in the Development Fund for Iraq must be shown to be used in Iraq's best interests.
It was understood that all revenues would be paid into a central fund.
But both the charity Christian Aid and the Liberal Democrats are now criticising the CPA, saying no audit of how the money was used had been carried out until April of this year - two months before the handover of power.
Christian Aid described the information regarding the allocation of money as "woefully inadequate".
On 29 May, the CPA revealed $19.4bn (£10.7bn) had been paid into the DFI and spent on a wheat purchase programme, electricity and oil infrastructure programmes and equipment for Iraqi security forces, among other purchases.
The Coalition said $10bn (£5.5bn) of the total sum came from oil revenues.
But, according to the Lib Dems, there is a shortfall of up to $3.7bn (£2.03bn) between the amount of oil revenue earned and the money paid into the DFI by the CPA.
Iraq's oil resources generate billions of dollars each year and both studies insist it is not known how that money has been spent.
Helen Collinson, from Christian Aid, said: "For the entire year that the CPA has been in power in Iraq it has been impossible to tell with any accuracy what the CPA has been doing with Iraq's money."
In a separate study, the Liberal Democrats said the CPA was obliged to pay all oil revenues into the DFI, but there appears to be a significant disparity.
According to CPA figures, the DFI had received $10.8bn (£5.9bn) for oil revenue by 21 June this year.
But research by the Lib Dems suggests oil revenues stand between $12.2bn (£6.7bn) and $14.5bn (£7.96bn).
Christian Aid put the figure at $13bn (£7.1bn).
Liberal Democrat foreign affairs spokesman, Sir Menzies Campbell, said: "This apparent discrepancy requires full investigation."
"Without concrete accounts from the CPA and the Iraqi transitional administration, questions will remain about how much has actually been earned and how much should rightfully be in the DFI."
The two studies will be published to coincide with the transfer of power on Wednesday.
Both investigations call on the CPA to reveal full details of expenditure and urge the British Government to exert some influence over the issue.
They also demand that a transparent system of financial accountability is put in place by the new Iraqi Government.