The absence of a handful of the world’s best swimmers, including Michael Phelps, has added an air of uncertainty to the 16th World Championships, starting in Russia tomorrow.

Taking place in Kazan from July 24-August 9, the World Cham-pionships were meant to be a full dress rehearsal for next year’s Olympics in Rio but a spate of high-profile withdrawals has diluted the competition.

The highest-profile no-show is the popular Phelps, who came out of retirement in 2014 but was subsequently banned from representing the United States at the World Championships meeting after being convicted of drink driving late last year.

Australia’s James Magnussen, who won the 100 metres freestyle gold at the last two worlds, will also be missing after electing to undergo shoulder surgery, giving up his chance to become the first man to win the blue-riband sprint three times in row.

France’s Yannick Agnel, the reigning world and Olympic champion for 200m freestyle, is also skipping the event, because of a lung infection.

The spotlight for the men’s events in Kazan is now likely to fall on South Africa’s Chad le Clos and China’s Sun Yang.

One of the few men to have beaten Phelps at the Olympics, Le Clos will defend the 100m and 200m butterfly titles he won at the 2013 World Championships in Bar-celona and is also targetting the 50 metres, hoping to complete a uni-que treble.

“This is the most important event outside of the Olympics, everything after this is just warm-ups,” the 23-year-old said.

American Katie Ledecky is trying to win four freestyle events in the Kazan meeting.

The 18-year-old has been smashing world records at will for the last year and seems a sure bet to win the three longer races, but the 200m is her toughest challenge.

Among Ledecky’s big rivals will be her team-mate Missy Franklin, who won four gold medals at the 2012 London Olympics as a 17-year-old.