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More than five trillion pieces of plastic - weighing as much as two large cruise liners - are floating in the world's oceans.

The total weight of all the plastic pollution in the seas is estimated to be almost 269,000 tonnes.

An international team of scientists made the calculation after gathering data from 24 expeditions mounted over a period of six years between 2007 and 2013.

The total weight of all the plastic pollution in the seas (stock image) is estimated to be almost 269,000 tonnes. An international team of scientists made the calculation after gathering data from 24 expeditions mounted over a period of six years between 2007 and 2013

Towed nets were used to scoop up plastic from five sub-tropical 'gyres' - huge areas of circulating ocean currents - as well as coastal Australia, the Bay of Bengal and the Mediterranean Sea.

Visual surveys provided information about large fragments of plastic material.

This data was combined, and a computer simulation of floating debris dispersal was used to indicate the oceans contain at least 5.25 trillion plastic pieces weighing 268,940 tonnes.

The plastic ranged from tiny particles less than a millimetre wide, to 'macro' fragments more than eight inches (20 centimetres) across.

Sub-tropical gyres are known to gather up plastic, but the research revealed that the rubbish was not confined to these ocean 'dustbins'.

The smallest particles were distributed to remote parts of the world, including sub-polar regions, suggesting that the gyres 'shredded' large plastic items and ejected the pieces.

Lead researcher Dr Marcus Eriksen, director of research at the Five Gyres Institute in Los Angeles, said: 'Our findings show that the garbage patches in the middle of the five sub-tropical gyres are not the final resting places for the world's floating plastic trash.

'The end-game for micro-plastic is interactions with entire ocean ecosystems.'

Large plastics appeared to be abundant near coastlines, the scientists found.

The amount of 'microplastic' particles on the surface of the oceans was much less than expected, suggesting that some of it was being removed.

Removal processes included degradation by sunlight, biodegradation, loss of buoyancy, entanglement with settling detritus, beaching, and ingestion by fish and other organisms.

Writing in the online journal Plos One, the scientists concluded: 'This is the first study that compares all sizes of floating plastic in the world's oceans from the largest items to small microplastics.

The amount of plastic on the surface of the oceans was much less than expected, suggesting that some of it is being removed. Removal processes included degradation by sunlight, biodegradation, loss of buoyancy, entanglement with settling detritus, beaching, and ingestion by fish and other organisms

Elsewhere, the bodies of sea turtles are being found with stomachs full of plastic, (plastic taken from the gut of a sea turtle corpse is pictured) because they are prone to mistaking bags for jellyfish. The plastic can block the digestive tract, leading to a slow death from starvation

'Plastics of all sizes were found in all ocean regions, converging in accumulation zones in the sub-tropical gyres, including southern hemisphere gyres where coastal population density is much lower than in the northern hemisphere.'

Albatrosses on remote pacific islands have been found to be ingesting so much plastic that their stomachs become full and they die. The sharp foreign objects also lacerate their gut walls.

Elsewhere, the bodies of sea turtles are being found with stomachs full of plastic, because they are prone to mistaking bags for jellyfish.

The plastic can block the digestive tract, leading to a slow death from starvation. 

It was recently estimated that 90 per cent of seabirds found dead on beaches have ingested plastic.