Prime Minister John Key has been bumped at the last minute from a worldwide televised climate change debate - in favour of Australian Prime Minister Kevin Rudd.

The BBC World news debate today - dubbed The Greatest Debate on Earth - has been billed as the media highlight of the Copenhagen climate talks and the Prime Minister had announced he would be in it.

John Key was told only shortly before he arrived in Copenhagen the evening before the debate that he was not going to take part. The show promises to challenge leaders over their commitment to fighting climate change.

Spokesman Kevin Taylor said it was disappointing and that the PM would have to re-structure his day, which had been arranged around the debate.

The BBC had earlier confirmed his place in the line-up but cancelled at the last minute, he said.

Asked why the BBC changed its mind, Mr Taylor said: "I have got an idea but I'm not going to do their explaining for them."

Mr Key will be replaced in the debate by Kevin Rudd, who will join Prime Minister of South Africa Jacob Zuma, President of Mexico Felipe Caldero and Indian Minister of Environment and Forests Jairam Ramesh.

Danish Prime Minister Lars Lokke Rasmussen, who will take over as conference president from former environment minister Connie Hedegaard as talks head into their final day, was tipped to attend but was not in the latest line-up this morning.

Greenpeace spokesman Geoff Keey said the slight was a sign that New Zealand was not really a player in the negotiations. "All the signals that New Zealand has been sending out this year is that we're not really serious about climate change."

Government ministers in Copenhagen had said when the debate was announced that New Zealand had been noticed partly because it was the only nation outside Europe with an all-sectors emissions trading scheme in place.

Labour climate change spokesman Charles Chauvel said when the debate was announced that the trading scheme would be difficult to defend on television.

The debate will take place at the DR Concert Hall near the Bella Centre starting at 5am on Friday, New Zealand time and be broadcast worldwide on the BBC World network after the climate summit ends on December 19.