Timberlake and Diaz both provide voices for the animated film

The actress again provides the voice of Princess Fiona, while the singer is heard as Artie, a new character.

The pair, who split up after three years in January, kissed on the cheek in front of hundreds of fans and cameras in London's Leicester Square.

Producer Jeffrey Katzenberg has promised "more Shreks are coming", with the fourth instalment filmed in 3D.

Diaz told BBC News 24 it was a privilege to be part of a film that made so many people laugh.

"We are like a family with Shrek," she said.

She put the success of the Shrek franchise down to the way it had redefined fairytales.

Diaz posed with Shrek on a special green carpet in Leicester Square

"It needed to be done. We get a parody of our fairytales and we have a lot of fun on it," she said.

Timberlake, who joined the third production already a fan of the films, said he had reverted to an eight-year-old during the recording.

"You are in a booth, just you and director. You are caught in this imaginary world."

Also at the British premiere on Monday evening was Mike Myers, who takes the lead as the voice of green ogre Shrek.

As part of their promotional tour, Myers, Antonio Banderas, Rupert Everett plus Diaz, attended a photocall at London landmark Tower Bridge and paid a surprise visit to the Great Ormond Street Hospital for Children.

They met some of the patients on the wards and posed for pictures with a life-sized Shrek character.

Mike Myers is the voice of Shrek

Last month Shrek the Third, which opens in the UK on 29 June, set a North American record for an animated film on its opening weekend, generating $120m (�61m).

The festive adaptation of the movie, Shrek the Halls, will feature the main characters' versions of winter traditions.

And one clue has been given by Katzenberg as to the plot of film number four.

He has said that Shrek will have to come to terms with something difficult in his own past.