The 34-year-old Foster has no plans to retire

The 34-year-old, who controversially missed out on Athens, came home first in 21.58 seconds to claim his fourth world gold.

"It is nice to be world champion again," said Foster, who also won gold in 1993, 1999 and 2000.

Foster, Britain's first gold medallist at these championships, also qualified for Sunday's 50m butterfly final.

"A lot of people have been telling me to finish on a high such as this but I still feel I can do well at the top level and while I can, I don't think I should stop," added Foster after his victory.

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"There were a lot of Olympic finalists and medallists there and I beat them, which gives me the buzz and the motivation to carry on."

Foster was not picked for Britain's team at the summer Olympics because he failed to make the qualifying time at the trials.

Britain had three other swimmers in finals on the third day of the five-day championships.

Teenager Cassandra Patten was fourth in the 400m freestyle in four minutes 9.20secs.

Alison Sheppard was fifth in the 100m individual medley in 1:01.51.

Meanwhile, Robin Francis, a silver medallist in the 400m individual medley, was seventh in the 200m individual medley in 1:58.80.

The US won four more golds on Saturday but were pipped in the women's 400m medley relay by Australia, who set a world record in the process.

The Australian team of Sophie Edington, Brooke Hanson, Jessica Schipper and Lisbeth Lenton won in 3:54.95 seconds, lowering Sweden's old mark of 3:55.78 set in 2002.