More than five trillion pieces of plastic weighing as much as two large cruise liners are floating in the world's oceans, a study has shown.

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

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.

Combined with the data, a computer simulation of floating debris dispersal indicated that the oceans contained at least 5.25 trillion plastic pieces weighing 268,940 tons.

The plastic ranged from tiny particles less than a millimetre wide to "macro" fragments more than 20 centimetres across.

Sub-tropical gyres are known to gather up plastic, but the research showed 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, 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."