The names of these new LNG-powered ships “contain words like ‘eco’ and they are often painted green,” said Transport & Environment.

“But their green credentials end there.”

Using a thermal imaging camera, the group said it filmed the emissions of two LNG-powered ships in the port of Rotterdam last November.

It said a review of the images by an independent optical gas imaging consultancy found that “intense uncombusted hydrocarbon emissions were being released into the atmosphere”.

The owner of one of the ships reviewed, French shipping giant CMA CGM, said it had “already identified the issue of uncombusted methane... and is working with its motor manufacturer partners” to reduce the emissions.

A spokesman said reductions in methane emissions had already been achieved.

Shipping companies began shipping to LNG primarily due to reduce high sulphur emissions from marine fuel, he also noted, as the pollution is a source of tensions with authorities in some port cities.