The initiative to write off debt owed by the world's poorest countries to the International Monetary Fund and World Bank has been held up by a dispute among rich nations on how much relief to grant.

The arguments centre around the effect of lower global interest rates on the debt relief calculations used by the heavily indebted poor countries (HIPC) programme.

They mean that the value of future debt payments in present value terms, discounted back to the present day using interest rates, are larger.

Officials familiar with the situation say that some countries on the IMF and World Bank boards, including the UK, France and Canada, have argued that the amount of debt relief on offer by the bank and fund should take account of this. Others, who they say include the US, Japan and Germany, have disagreed.

The US Treasury said it was concerned that countries would use increased debt relief as an excuse to borrow more from the bank. "We are concerned about the bank being in a position where it is in a continual cycle of lending and forgiving," said a US Treasury official.

This dispute has already arisen with the west African country of Niger - made famous by the dispute over whether Saddam Hussein, the former president of Iraq, tried to buy uranium from the country.

Niger has also been hit by falling world uranium prices which have damaged its ability to hit key debt-to-export ratios, a slide in the volume of uranium exports slides and a projected drying-up of aid from rich countries, particularly the European Union.

IMF-World Bank staff calculations suggest Niger will have a debt-to-export ratio of 200 per cent without "topping up", or increasing the amount of relief halfway through the process.

This would leave it well in excess of the 150 per cent target set by the HIPC programme, officials familiar with the study said.

Executive board meetings of the IMF and World Bank to discuss debt relief for Niger were postponed last month when it became clear that agreement could not be reached on the amount of topping-up necessary, according to the officials.

The amount of topping-up under question, $142.5m (?112m, £77m) in net present value terms, is sizeable in comparison with Niger's overall $500m in promised debt relief. The same issue will come up repeatedly with each country finalising its debt relief arrangements. Next in line is Ethiopia.

Campaigners complain that the fund and bank are dragging their feet on granting debt relief to Ethiopia. "By bending over backwards to cancel Iraq's debt, and at the same time wilfully flouting their own commitments to Ethiopia, creditors are breaking promises to their electorates, as well as undermining progress in heavily indebted countries," said Ann Pettifor, director of Jubilee Research at the New Economics Foundation in London.

The HIPC programme applies to around 40 of the world's poorest countries, mainly in sub-Saharan Africa. Development campaigners have complained that criteria for calculating relief were arbitrary and inadequate.