Prince Charles will say that the need to tackle global warming is more urgent than ever before and that, even in a global recession, the world must not lose sight of the "bigger picture".

His warning will be delivered on Thursday in a keynote speech in Rio de Janeiro.

Aides believe it will echo one he gave in Sao Paulo in 1991 at the start of the last recession, when he warned that caring for the world's long term welfare must not become a "luxury".

The intervention will help to put the environment at the top of the political agenda ahead of the meeting of G20 leaders in London next month.

The Prince starts a ten-day tour of South America today during which he will be playing an elevated role as an international statesman working on behalf of the Government to support British interests on key issues.

Senior sources have revealed that Gordon Brown's Government wants to make more use on the foreign stage of Prince Charles's experience, expertise and contacts, particularly on climate change.

Government officials believe that the Prince's passion to protect the environment is hugely respected abroad and that he can play an increasing important role as he inevitably moves closer to becoming king.

Some believe he is an "asset" that has been underused in the past and they want to use him more in a role of "soft diplomacy".

In Thursday's speech, the Prince will warn that a failure to act in the next eight years will have catastrophic effects for the planet.

In the country that is home to the world's largest rainforest, Prince Charles will urge world unity to combat deforestation in the run-up to the UN conference on climate change in Copenhagen in December this year.

He will say there is not necessarily a clash between the needs of big business and the environment. He will argue that being green can be good for businesses and can create jobs.

The Prince will say that tackling deforestation in the 3.5 billion acres of rainforest on the planet is a key priority.

According to royal aides, Prince Charles will tell business leaders that he is in South America to "listen and learn" and that he believes that Brazil and other neighbouring countries must lead the way in moves to save the rainforests.

The Prince will also visit the Amazon rainforest, the world's largest rainforest, on Saturday.

The Prince's tour to Chile, Brazil and Ecuador (including the Galapagos Islands), when he will be accompanied by the Duchess of Cornwall, will see a greater emphasis than ever before on him becoming involved in talks with prime ministers, presidents and senior politicians, in a move aimed at promoting British interests abroad.

It is the Foreign and Commonwealth Office (FCO), not the Prince himself, which chooses the location for his royal tours and South America is considered the ideal platform for his views.

One Government source said: "The Prince's visit to South America is very much in tune with the priorities that we have.

"Given both the Prince's position as a future head of state and his personal commitment to protecting the environment, we believe he can have a real impact abroad"

In a possible sign of his elevated role, Prince Charles will meet Barack Obama when the new US President comes to Britain next month for the G20 London summit.

They will meet at, or even before, a Buckingham Palace reception when the Prince is expected to raise climate change issues with the President.

Royal aides have welcomed the opportunity for the Prince to expand his role on behalf of the Government. One senior official said: "It could be argued that he has already established a role as an international statesman, especially on green issues.

"This is one of the reasons why Government has asked him to go to South America. He will be conducting Government business and one of his roles is to support the UK's environmental objectives."

This is the major royal overseas tour for the first half of the year. He will be meeting the presidents and heads of governments in each of the three countries for talks.

"In Chile and Brazil, he will host a "round table" meeting for corporate leaders aimed at encouraging businesses to do more to tackle climate change.

"This is about soft diplomacy," said a senior aide. "The Prince of Wales is not a politician who sits down with a world leader and hammers out an agreement.

"But what he can do is show support and get over the message that Britain wants to stand shoulder to shoulder with these countries in the fight against climate change, and bring his own experience and knowledge to bear."

Courtiers at Clarence House dislike suggestions that the Prince of Wales, who was 60 last year, is a "shadow king" keen to take on duties from his mother, who is 82.

However, one Government source said: "There is no plan for Prince to take over specific duties from the Queen but common sense dictates that as she gets older her diary will be tailored accordingly."

The Prince is likely eventually to take on more of the long, arduous overseas tours stand in for her at more investitures.

One senior FCO official said: "We are hugely supportive of this visit. The Prince's commitment to sustainable development is very much in line with one of our main priorities.

"The Prince's visit will advance that agenda in terms of both UK interests and the engagement of other countries in pursuing it.

"The real personal commitment that Prince Charles has to environmental issues and his interest in sustainable development really resonates with people, both at Government level and ordinary individuals."

by:telegraph