Olympics officials are to drain one of the swimming pools that turned green and refill it in a bid to revert it to its usual colour.

Almost a million gallons of water in the pool at Rio's Maria Lenk Aquatics Centre change colour, leaving athletes baffled.

Officials said they would be draining the pool a matter of hours before the synchronised swimming events started today.

And they also revealed the suspected reason behind the change in colour.

Rio 2016's director of venue management Gustavo Nascimento said: "On the day of the Opening Ceremonies of the Games, 80 litres of hydrogen peroxide was put in the water.

"This creates a reaction to the chlorine which neutralises the ability of the chlorine to kill organics. This is not a problem for the health of anyone."

Hydrogen peroxide is used as a bleaching agent and disinfectant.

Mr Nascimento added: "We have been trying to clean the pool for four days, and obviously it's not going as fast as we want it to.

"We are going to grab the water from the warm-up pool, drain the water out of the competition pool and insert the water from the warm-up pool into the competition pool. The water in the warm-up pool is in perfect condition."

Rio 2016 spokesman Maria Andrada added: “Of course it is an embarrassment because we are hosting the Olympic Games.

"It should be light blue, transparent. We could have done better in fixing it quickly. We learned a painful lesson the hard way.”