ATHENS — Life in the overcrowded migrant camps on Greek islands “has dramatically worsened” in the past year, with thousands of refugees fighting to meet their most basic needs, Europe’s top human rights official warned on Thursday.

“It is an explosive situation,” the official, Dunja Mijatovic, the human rights commissioner of the Council of Europe, said after going to several camps. For many people in the camps, she said, “this has become a struggle for survival.”

The Greek Parliament was expected to adopt a bill later Thursday aimed at easing crowding in the camps. But that plan could simply substitute one set of problems for another, Ms. Mijatovic suggested .

“There is a desperate lack of medical care and sanitation in the vastly overcrowded camps I have visited,” she said at a news conference in Athens. “People queue for hours to get food and to go to bathrooms, when these are available.”