Prolonged exposure to crude oil and chemical dispersants is a public health danger, environmental scientists warned yesterday as BP spent a third day trying to initiate a “top kill” operation to cap the ruptured well on the sea bed.

The oil firm moved to a second stage of the procedure by injecting material such as golf balls, shredded tyres and rope into the well. But John Pack, a spokesman for BP, said it would not be clear until tomorrow if it would work. “We have never said there is a deadline or a schedule,” he said. “We need to take this pretty slowly, but everything is going according to plan.”

BP’s beleaguered chief executive, Tony Hayward, yesterday drastically scaled upwards his assessment of the spill in the Gulf of Mexico. “This is clearly an environmental catastrophe. There is no two ways about it,” he told CNN. “It’s clear that we are dealing with a very significant environmental crisis and catastrophe.” In an interview with the Guardian two weeks ago, he had described the oil spill as “tiny” relative to the size of the gulf.

“The amount of volume of oil and dispersant we are putting into it is tiny in relation to the total water volume,” he said then.

However he was optimistic that the “top kill” stood a good chance of success, which he put at 60-70%. “We have wrestled it to the ground, but we haven’t put a bullet in its head yet,” Hayward said.

With no immediate end in sight, there were growing concerns over the effects on public health of a prolonged exposure to the oil as well as to the more than 3,640,000 litres (800,000 gallons) of chemical dispersants sprayed on the slick.