Madonna's performance at the Eurovision Song Contest on Saturday has been thrown into doubt by organisers who say she has not yet signed a contract for the show.

In April, it was announced that the pop superstar would perform two songs at the grand final in Tel Aviv in Israel on Saturday, May 18. 

However, the show's organisers have now cast doubt on her performance.

Eurovision's executive supervisor Jon Ola Sand said on Monday during a press conference: "The European Broadcasting Union has never confirmed Madonna as an act.

"If we do not have a signed contract she cannot perform on our stage."

Sand, who has produced the song contest since 2010, said that negotiations with Madonna are still ongoing.

"We are in a situation now that is a bit strange," he said.

"We have an artist who would like to participate in the Eurovision Song Contest, and who we would love to welcome on that stage. But for that we need to have the framework secured.

"We are negotiating now, in the final stage of that - but if there is no signed contract this week, she will not be on the stage."

On Tuesday, Madonna defended her decision to perform at the song contest in Israel following calls for a boycott of the competition by pro-Palestinian activists who want companies, performers and governments to disengage from Israel.

"I'll never stop playing music to suit someone's political agenda nor will I stop speaking out against violations of human rights wherever in the world they may be," the singer said in a statement to Reuters.

"My heart breaks every time I hear about the innocent lives that are lost in this region and the violence that is so often perpetuated to suit the political goals of people who benefit from this ancient conflict.

"I hope and pray that we will soon break free from this terrible cycle of destruction and create anew path towards peace."

Israel is hosting the Eurovision contest after singer Netta Barzilai won last year.