The use of wetsuits had not been mentioned in the governing body's previous rules, but FINA have given swimmers the all clear to wear them in the event the temperature is below 20 degrees celsius.

Athletes will have the choice of either a racing suit or wetsuit in that instance, with the latter determined to provide "thermal insulation".

In the event the temperature is two degrees colder, swimmers will be required to use the wetsuit for safety reasons, which will completely cover their torso, back, shoulders and knees, but will not extend beyond the neck, wrists and ankles.

The governing body has also determined that swimming caps which meet pool specifications are not acceptable for open water competitions.

In team open water races, swimmers are required to wear the same colour hat as the other members of their team.

A bylaw concerning team relays has also been added, with the new rules coming into force yesterday.

Rather than the existing format of World Championship open water relays, which featured two men and one woman, there will now be two swimmers from each sex in a four person event.

All four swimmers will need to complete one lap, consisting of 1.25 kilometres, while there will be a "changeover zone".

"Changeovers shall be made with the following swimmer in the water in their starting position holding the platform," the FINA bylaw states, adding that "changeovers shall be made with an obvious visible touch between the swimmers making the change".

Swimmers may enter the change-over zone when the previous swimmer is approaching the zone and leave the water immediately at the conclusion of their lap.

While the new bylaws mainly effect open water swimming, the governing body have also made a minor change to synchronised swimming rules.

They have stated that "swimwear for men in synchronised swimming shall not extend above the navel nor below the upper thigh".

Male synchronised swimmers are not allowed to wear make-up, although hair gel and moustaches have been permitted.