In a speech on Tuesday to the UN General Assembly, Italian ambassador Marcello Spatafora accused Germany, Japan, India and Brazil - aspirants for permanent council seats known as the Group of Four, or G4 - of "blackmailing some sector of the membership".
He referred to the G4 as a unit without making it clear which country or countries he meant.
"Enough is enough," Spatafora told the assembly. "I am referring to the G4 resorting to financial leverage and to financial pressures in order to induce a government to align or not to align itself with a certain position."
Spatafora gave one example of an unnamed G4 donor country allegedly threatening a government co-sponsoring his draft resolution, that it would "put an end" to a $460,000 development project for children.
Spatafora accused the group of using aid money to try to buy seats on the UN Security Council and demanded an investigation of the "improper and unethical behaviour".
He warned that using development aid to win support from poor countries was tantamount to blackmail.
He said it could spark a scandal that would be "much more serious and destabilising" than the controversy surrounding the UN oil-for-food programme in Iraq.
"Here it is not a question of pocketing money," he said. "It is a question of ethics and moral values."
Kofi Annan has been asked to
investigate G4 nations
Spatafora called on General Assembly President Jean Ping and Secretary-General Kofi Annan to set up an investigation, not "sweep the dust under the carpet" and accept "a stained reform" of the UN's most powerful body.
However, the head of economic affairs in Japan's UN Mission said Tokyo's development assistance was based on the plans and strategy of the developing countries.
"I think this is completely groundless," Kazuo Sunaga said of the Italian ambassador's accusations.
Calls to the German, Brazilian and Indian missions were not immediately returned.
Italy has long campaigned against adding new permanent members to the council. A seat for Germany would leave Rome as the only big Western European nation without one.
Spatafora spoke after Canada, backed by Pakistan and Colombia, introduced a resolution, the third plan to date, to expand the council by 10 new rotating seats and no permanent seats, as the G4 wants.
"It is a question of ethics and moral values"
Italy's ambassador to the UN
About a dozen countries are part of the group backing this proposal, called "Uniting for Consensus".
The contentious struggle for Security Council reform has been debated for a decade and given momentum this year by Annan, who argues the council is unrepresentative and reflects the balance of power at the end of the second world war.
Germany, Japan, Brazil and India have called on the General Assembly to enlarge the Security Council to 25 from 15. This plan envisions six new permanent seats, including two for Africa, but new members would not have veto power.
The Security Council now has 15 seats, 10 rotating for two-year terms and five permanent members with veto power: Russia, the United States, China, Britain and France.
The last step in council reform would be approval by national legislatures
and here the five can use their veto.