Forty percent of the world's six billion people still lack sanitation though it could easily be provided, according to a United Nations report issued today.

More than a billion people lack the most basic water supply, said the study, backed by the World Health Organization and the United Nations Children's Fund.

''It is not a question of cost but of priority,'' said Richard Jolly, chairman of the Water Supply and Sanitation Collaborative Council, sponsored by the W.H.O.

Bringing water and sanitation to all would cost $10 billion a year, Mr. Jolly said. That, he added, is ''one-tenth of what Europe spends on alcoholic drinks each year, about the same as Europe spends on ice cream and half of what the United States spends each year on pet food.''