Reports by Christian Aid and the Liberal Democrats, Britain's third-largest political party, said there were glaring gaps in the handling of $20 billion generated by Iraq's oil.
The Christian Aid report on Sunday also said the majority of Iraq's reconstruction projects had been awarded to US companies, which charged up to 10 times more than their Iraqi equivalents.
The UN gave the US occupation authority responsibility for the Development Fund for Iraq (DFI) after the fall of Saddam Hussein in May 2003.
But it stipulated that expenditure must be shown to be in the country's best interests and that all revenue should be paid into a simple fund.
However, Christian Aid and the Liberal Democrats said that no audit on the money had been carried out until April 2004.
Christian Aid spokeswoman Helen Collison said that for an entire year it "has been impossible to tell with any accuracy what the CPA [Coalistion Provisional Authority] has been doing with Iraq's money".
The occupation authority reported in May that $9.4 billion had been paid into the DFI and spent on a wheat purchase programme, electricity and oil infrastructure and equipment for Iraqi security forces.
It "has been impossible to tell with any accuracy what the CPA [Coalistion Provisional Authority] has been doing with Iraq's money"
Christian Aid spokeswoman
It said that $10.8 billion of the total sum was due from oil revenues by 21 June, this year.
However, the Liberal Democrat report said its research suggested that oil revenues stood at $12.2 billion to $14.5 billion. Christian Aid put the figure at $13 billion.
Lack of clarity
Both reports stressed it was not clear how much of the money had been spent. The Liberal Democrats' study cited the accounting firm KPMG, which criticised the occupation authority for not metering oil production and questioned its spending.
"This apparent discrepancy requires full investigation," Menzies Campbell, the Liberal Democrat spokesman for foreign affairs, said.
"The cost of reconstruction of Iraq is considerable and those countries who are being asked to contribute will want to know that Iraq's own resources are making a maximum contribution."
Christian Aid said that Iraqi companies had been awarded contracts worth a miniscule total of $500,000 since April 2004.
The report also claimed around $2 billion of Iraqi money was given to mostly US companies for poorly thought-out projects of no lasting benefit.