The G8's plan to help 18 of the poorest nations may not go ahead if non-G8 countries are not included, says the UK International Development Secretary.
These demonstrators are calling for the G8 deal to be ratified
The G8 - the UK, Canada, France, Japan, Germany, Italy, Russia and the US - agreed the deal at Gleneagles in July.
They aim to axe $40bn (£22bn) debt owed to international lenders by a group of impoverished nations, mostly in Africa.
But Hilary Benn said the G8 still had to persuade others "this is a good deal for developing countries".
A number of countries, led by Belgium and the Netherlands, have complained that not enough money has been committed by rich nations to make up for the shortfall in funds that the World Bank will face if debt repayments are forgiven.
"The deal done in Gleneagles was a deal of the G8 countries, and there are others that need to come in, and understandably they have asked questions about it," Mr Benn said.
He is attending talks with the International Monetary Fund (IMF) and the World Bank, at a gathering of global lenders in Washington.
Financial leaders are due to discuss a range of issues, from poverty to the global impact of higher oil prices.
Officials representing the 184 members of the World Bank and IMF are under increasing pressure to back the debt relief deal struck by the G8.
Mr Benn told BBC Radio 4's Today programme: "It is a very, very important deal. But there are other countries represented on the World Bank board in particular and they need to support it too."
He said within the World Bank, IMF and African Development Bank there was basic agreement that the deal must proceed.
He added: "What I think people will not want to see happen is at the end of this weekend some people to have got in the way of a deal.
"The G8 is solid and we have got to persuade the other countries that this is a good deal for developing countries."