Australia's media watchdog has launched a formal inquiry into the prank call to the London hospital where Kate Middleton was being treated.

Nurse Jacintha Saldanha was found dead on Friday, three days after taking the call from an Australian radio station.

The Australian Communications and Media Authority said it would focus on the 2DayFM licence holder and not the two presenters who made the call.

It will be examining if "broadcasting obligations" were breached.

Ms Saldanha put the prank call through to a colleague who disclosed details of the treatment being given to Ms Middleton, who is suffering from acute morning sickness.

The incident made headlines around the world, as did news that Ms Saldanha, who was married with two children, had then apparently taken her own life.

British lawmaker Keith Vaz, who represents Ms Saldanha's family, said he had written to Southern Cross Austereo, parent company of Sydney radio station 2Day FM, to express his dissatisfaction.

"There has been no written apology, no request for a meeting with the family and no attempt to travel to the United Kingdom to express contrition," Mr Vaz wrote in a letter to Southern Cross chief executive Rhys Holleran that he released to the media.

Southern Cross, which has apologised for the stunt, said on Tuesday it would donate its advertising revenue until the end of the year to a fund for Ms Saldanha's family, with a minimum contribution of $500,000 (€400,000).

"I would be grateful if you could let me know how you arrived at this figure and why you think this adequately deals with this serious and important issue," Mr Vaz wrote.

Southern Cross and its two presenters, Mel Greig and Michael Christian, have faced a barrage of criticism since Ms Saldanha, 46, was found dead in staff accommodation near London's King Edward VII hospital.

Ms Greig and Mr Christian have both been suspended and their show has been scrapped. They appeared on Australian television to say the death had left them heartbroken.