Williams made his comments on a trip to Mexico

"I can't believe he got back in again," said the musician at a press conference on a promotional visit to Mexico.

"I'm scared as an individual, I'm scared for the world. The guy is obviously an idiot," added the star.

The 30-year-old said that his public speaking skills were better than those of Mr Bush, and that he would do a better job as US president.

"Americans keep saying they're the leaders of the free world, and this guy can't form a sentence," he said.

Critical

"I speak better than him, you know. I'd do a better job than him. And I'm not very bright, I'm not," added the chart-topper.

Williams was in Mexico to promote his greatest hits album, and is due to visit Brazil next - but no live performances are scheduled.

The singer participated in the recording of the new Band Aid 20 record, and was recently inducted into the UK Music Hall of Fame.

He was voted the most influential artist of the 1990s in a TV poll, joining automatic qualifiers Madonna, The Beatles, Elvis Presley, Bob Marley and U2.