Many navies are shrinking as defence cuts bite, but environment groups are renewing their fleets in response to growing ecological pressure on oceans and losses of their vessels at sea.

German and Polish shipyards will shortly start work on Greenpeace's GBP14m flagship, a mega-yacht that will become the third Rainbow Warrior next year.

It will be one of the biggest yachts to have been commissioned in the past decade with, say the designers, a massive 1300 sq m of sail supported on two A-frame masts.

The Rainbow Warrior III will have its own helipad and room for a flotilla of inflatables.

But while it will sleep 30 in more comfort than the fishing boats the environment group usually converts, it will be one of the greenest ships afloat and a satellite system will allow campaigners to stream video footage from anywhere in the world.

"We have converted ships for 30 years and it's time we practised what we preach," said Ulrich von Eitzen, a Greenpeace spokesman.

"Upgrading the existing ship was not technically or financially feasible and converting a secondhand ship would compromise our campaigning and energy conservation needs."

The new ship will have both diesel and electric engines but these are expected to be in use for less than 10 per cent of its time at sea.

"The aim is to drastically reduce emissions and to burn far less fuel. You can never say in advance what speeds it will do, but its main propulsion will be by wind," Mr Eitzen said.

Analysis showed that it was ecologically efficient to build from new rather than convert another ship, but that there was little difference between an aluminium or steel hull.

By far the greatest impact on the environment was found to be during the ship's use rather than in the construction or eventual demolition phases.

The decision to choose sail rather than rely on fossil fuels is also a deliberate challenge to the shipping industry to reduce its carbon and other polluting emissions.

Ships' diesel engines rely on "bunker" oil, one of the dirtiest fuels in the world, but owners are coming under increased pressure to reduce emissions and marine designers are investigating a return to sail.

Critics have argued that Greenpeace has been reluctant to address shipping emissions for fear of drawing attention to its own fossil-fuel-powered fleet.

The cost should not be a problem for the group which, with nearly three million supporters, is extremely wealthy.

The new Rainbow Warrior will bring the Greenpeace fleet to six ocean-going ships, as big as the navies of many island states such as Madagascar, the Seychelles, the Maldives and Mauritius.

Greenpeace boats have been used from the Arctic to the Amazon to confront whalers, loggers, illegal fishers, GM food importers and nuclear testing.

The Sea Shepherd conservation group, based in California, is also planning to augment its fleet of former US and British coastguard ships after one of its vessels was smashed in half by a Japanese whaling ship in Antarctica the week before last.