Williams posted a time of 11.40sec as she led home America's Takeia Pinckney and Jamile Samuel of Holland.

The 16-year-old from Welwyn Garden City in Hertfordshire is the UK junior record holder and current world number three, and will bid to make it a sprint double when she begins the 200m heats tomorrow.

Williams had progressed with ease through her semi-final earlier in the evening in 11.59, the quickest of the qualifiers, before adding the World Junior title to her 2009 World Youth gold.

"I needed to get a good start because I knew both Americans could come through at the end," she said.

"I thought that if I got off to a good start I'd be in with a good chance and that's just what I did.

"I knew it was going be a close race and I wasn't quite sure I could come out and perform again, so it's obviously a huge relief and the pressure has lifted a bit."

Williams has not been beaten since she took up the sport five years ago, although she is not getting carried away by her record.

"I know the day I get beaten will come eventually, but my winning streak has given me great confidence coming here and getting a medal is brilliant," she said.

"This is another step up for me.

"The next step is the World Seniors so this is a stepping stone but it means a lot, it's a great achievement."

The men's race was won by Jamaica's Dexter Lee, who became the first sprinter to win back to back titles.

The 19-year-old, who is a full-time athlete, was involved in a very tight finish before being given the verdict ahead of Charles Silmon with Jimmy Vicaut third.

Lee was trailing his American rival at 80m put powered through to win by 0.02 with a time of 10.21 from Silmon who posted a personal record with France's Vicaut third in 10.28.