Brazil's Cesar Cielo Filho celebrates after winning the men's 50 meters freestyle final at the Paris Open swimming competition June 26, 2011. [Photo/Agencies]

The world swimming championships will open under a cloud in Shangahi on Saturday with a potential doping suspension still hanging over world and Olympic freestyle champion Cesar Cielo.

The 14th edition of the biennial competition, which will provide a pointer towards the athletes to watch at next year's London Olympics, have been shrouded in controversy since Cielo and three other swimmers tested positive for the banned diuretic furosemide after the Brazilian national championships in May.

The 24-year-old Cielo, Nicholas Santos, Henrique Barbosa and Vinicius Waked, all escaped censure from their national body, but world governing body FINA appealed that decision to the Switzerland-based Court of Arbitration for Sport (CAS).

CAS said on Wednesday that they would hear the appeal on July 20 at a special meeting in Shanghai, with its final decision due no later than July 22, just two days before the marquee pool competition begins.

Cielo, who won gold in the 50 and 100 metres freestyle at the 2009 Rome world championships and is the Olympic 50 metres freestyle champion, said the positive test had been caused by a supplement he took regularly that had become contaminated.

Cielo is also the world record holder in the 50 and 100 freestyle and had set the fastest time in the 50 freestyle and second fastest in 100 this year, with storming swims in Paris late last month.

Aside from the controversy around Cielo, the swimmers will be looking to put to bed the swimsuit issue that dominated the last championships in Rome, when 43 world records were broken in the pool.

FINA eventually banned the suits, which improved buoyancy and speed, and few people have approached the marks set either in Rome or in the time before the ban came into effect on Jan. 1 2010 and few expect too many of the records to be approached in Shanghai.

Dominate the Diving

American Michael Phelps, who along with Jamaican sprinter Usain Bolt dominated the headlines at the Beijing Olympics in 2008, returns to China with a less demanding, though no less ambitious, programme.

Phelps, who won an unprecedented eight gold medals in Beijing, is expected to race in the 200 metres freestyle, 100 and 200 butterfly and 200 individual medley and probably three relays.

The gruelling 400 individual medley has slipped away, with compatriot Ryan Lochte and Hungary's Laszlo Cseh battling to take Phelps' mantle.

More than 2200 athletes from 181 countries are expected in Shanghai for the 16-day event, which begins on Saturday with the women's three-metres springboard synchro -- the first medal to be decided.

China is expected to dominate the diving programme, though British teenager Tom Daley, who won the 10-metre platform event in Rome in 2009 at the age of 15, could prove tough to beat as he looks to build towards winning gold at his home Olympics next year.

His chances of defending the title improved with the late withdrawal by Australia's Beijing Olympic champion Matthew Mitcham due to an abdominal injury.

The synchronised swimming programme begins on Sunday with the Russians expected to continue their dominance.