Cruise ships have doubled in size over the past two decades, a precedent that could lead to even bigger vessels plying our oceans in the future and bringing potential problems, a new report has warned.

The largest passenger vessels today are twice as large as they were in 2000, says a study conducted by European clean energy lobby group Transport and Environment (T&E), which warns of the environmental impact of the "rapidly" growing global cruise industry.

The report suggests that the biggest cruise ships setting sail in 2050 could be nearly eight times larger than the Titanic, the largest passenger ship in service when it set sail in 1912 (measuring 269 meters or about 883 feet), if the growth rate continues as it is.