A study by the World Wildlife Fund (WWF), to be released on Tuesday, warns that the human race is plundering the planet at a pace that outstrips its capacity to support life.

In a damning condemnation of Western society's high consumption levels, it adds that the extra planets (the equivalent size of Earth) will be required by the year 2050 as existing resources are exhausted.

The report, based on scientific data from across the world, reveals that more than a third of the natural world has been destroyed by humans over the past three decades.

Using the image of the need for mankind to colonise space as a stark illustration of the problems facing Earth, the report warns that either consumption rates are dramatically and rapidly lowered or the planet will no longer be able to sustain its growing population.

Experts say that seas will become emptied of fish while forests - which absorb carbon dioxide emissions - are completely destroyed and freshwater supplies become scarce and polluted.

The report offers a vivid warning that either people curb their extravagant lifestyles or risk leaving the onus on scientists to locate another planet that can sustain human life. Since this is unlikely to happen, the only option is to cut consumption now.

Systematic overexploitation of the planet's oceans has meant the North Atlantic's cod stocks have collapsed from an estimated spawning stock of 264,000 tonnes in 1970 to under 60,000 in 1995.

The study will also reveal a sharp fall in the planet's ecosystems between 1970 and 2002 with the Earth's forest cover shrinking by about 12 per cent, the ocean's biodiversity by a third and freshwater ecosystems in the region of 55 per cent.

The Living Planet report uses an index to illustrate the shocking level of deterioration in the world's forests as well as marine and freshwater ecosystems. Using 1970 as a baseline year and giving it a value of 100, the index has dropped to a new low of around 65 in the space of a single generation.

It is not just humans who are at risk. Scientists, who examined data for 350 kinds of mammals, birds, reptiles and fish, also found the numbers of many species have more than halved.

Martin Jenkins, senior adviser for the World Conservation Monitoring Centre in Cambridge, which helped compile the report, said: 'It seems things are getting worse faster than possibly ever before. Never has one single species had such an overwhelming influence. We are entering uncharted territory.'

Figures from the centre reveal that black rhino numbers have fallen from 65,000 in 1970 to around 3,100 now. Numbers of African elephants have fallen from around 1.2 million in 1980 to just over half a million while the population of tigers has fallen by 95 per cent during the past century.

The UK's birdsong population has also seen a drastic fall with the corn bunting population declining by 92 per cent between 1970 and 2000, the tree sparrow by 90 per cent and the spotted flycatcher by 70 per cent.

Experts, however, say it is difficult to ascertain how many species have vanished for ever because a species has to disappear for 50 years before it can be declared extinct.

Attention is now focused on next month's Earth Summit in Johannesburg, the most important environmental negotiations for a decade.

However, the talks remain bedevilled with claims that no agreements will be reached and that US President George W. Bush will fail to attend.

Matthew Spencer, a spokesman for Greenpeace, said: 'There will have to be concessions from the richer nations to the poorer ones or there will be fireworks.'

The preparatory conference for the summit, held in Bali last month, was marred by disputes between developed nations and poorer states and non-governmental organisations (NGOs), despite efforts by British politicians to broker compromises on key issues.

America, which sent 300 delegates to the conference, is accused of blocking many of the key initiatives on energy use, biodiversity and corporate responsibility.

The WWF report shames the US for placing the greatest pressure on the environment. It found the average US resident consumes almost double the resources as that of a UK citizen and more than 24 times that of some Africans.

Based on factors such as a nation's consumption of grain, fish, wood and fresh water along with its emissions of carbon dioxide from industry and cars, the report provides an ecological 'footprint' for each country by showing how much land is required to support each resident.

America's consumption 'footprint' is 12.2 hectares per head of population compared to the UK's 6.29ha while Western Europe as a whole stands at 6.28ha. In Ethiopia the figure is 2ha, falling to just half a hectare for Burundi, the country that consumes least resources.

The report, which will be unveiled in Geneva, warns that the wasteful lifestyles of the rich nations are mainly responsible for the exploitation and depletion of natural wealth. Human consumption has doubled over the last 30 years and continues to accelerate by 1.5 per cent a year.

Now WWF wants world leaders to use its findings to agree on specific actions to curb the population's impact on the planet.

A spokesman for WWF UK, said: 'If all the people consumed natural resources at the same rate as the average US and UK citizen we would require at least two extra planets like Earth.'

The world's ticking timebomb

Marine crisis:

North Atlantic cod stocks have collapsed from an estimated 264,000 tonnes in 1970 to under 60,000 in 1995.

Pollution:

The United States places the greatest pressure on the environment, with its carbon dioxide emissions and over-consumption. It takes 12.2 hectares of land to support each American citizen and 6.29 for each Briton, while the figure for Burundi is just half a hectare.

Shrinking Forests:

Between 1970 and 2002 forest cover has dwindled by 12 per cent.

Endangered wildlife:

African elephant numbers have fallen from 1.2 million in 1980 to half a million now. In the UK the songbird population has fallen dramatically, with the corn bunting declining by 92 per cent in the past 30 years.