Carlos Nagarinos, director of the United Nations Industrial Development Organisation (UNIDO), was speaking ahead of the agency's annual conference on Monday, which is set to focus on that issue.
Nagarinos told reporters that recent figures showed that the number of people subsisting on less than one dollar a day in the 39 least developed countries had jumped from 125 million in 1970 to 280 million now.
At the same time, these countries' share in worldwide exports had decreased from 4.7% to less than 1%, while their share in global investments stood at merely 0.5%, he said.
"This marginalisation of less developed countries is unacceptable," Nagarinos said, adding that in African countries, the number of "very poor" people had jumped from 55.8% to 64.9% of the population.
"In order to help developing countries to become less dependent, it is vital we back companies in their bid to negotiate and build on industrial projects," Nagarinos said.
UNIDO's 10th annual conference opens on Monday in Vienna and runs through to Friday.
Several heads of state and government, as well as industry and trade ministers, are due to attend the conference.
The star speaker at the conference is to be Jeffrey Sachs, considered one of the world's foremost economists and the director of the Earth Institute.
He is also special adviser to UN Secretary General Kofi Annan on a group of poverty alleviation initiatives, called the Millennium Development Goals.
"In African countries, the number of "very poor" people had jumped from 55.8% to 64.9% of the population"
"UNIDO's conference will focus on a new development agenda based on productivity enhancement in areas including trade..., post-conflict reconstruction, and poverty eradication," said Magarinos.
Yukio Takasu, Japan's permanent representative to UNIDO, said the agency was "indispensable as a global forum."
UNIDO has 117 member states and is represented in 35 countries where it has committed about $49 million for development projects in the last two years.