Greta Thunberg's trans-Atlantic voyage on a 'zero-carbon yacht' has been rocked by revelations that crew will fly to New York in a gas-guzzling plane to bring the boat back to Europe.

It is claimed that this would generate more emissions than the yacht saves and threatens to leave the 16-year-old's plans to chart an environmentally friendly route to the United States in tatters.

On Wednesday, the Swedish eco-campaigner left Plymouth on the Malizia II for a two-week journey to the United Nations headquarters where she will address a climate change meeting.

But last night, it was confirmed that two crew will have to fly to the US east coast city to man the 60ft yacht on its return.

Greta Thunberg, 16, is sailing across the Atlantic in a 'zero-carbon yacht' to chart an environmentally friendly route to the UN climate conference in New York

But is has been revealed that two crew will fly to New York to bring the boat back, and it is also expected that skipper Boris Herrmann (left) and Team Malizia founder Pierre Casiraghi (middle) will fly back to Europe

On Wednesday, the Swedish eco-campaigner left Plymouth on the Malizia II for a two-week journey to the US east coast

'We added the trip to New York City at very short notice, and as a result two people will need to fly over to the US in order to bring the boat back,' a Team Malizia spokeswoman told the Times.

She added: 'The world has not yet found a way to make it possible to cross an ocean without a carbon footprint.'

And a further two sailors who are currently on board the Malizia II with Greta may use air travel to get back to Europe.

Skipper Boris Herrmann and Pierre Casiraghi, the nephew of Prince Albert of Monaco, will reportedly be boarding flights for their return trip.

Greta, who is taking a sabbatical year from school, will be joining large-scale climate demonstrations and speaking at the UN Climate Action Summit hosted by secretary-general Antonio Guterres in New York in September.

Teenage climate change activist Greta Thunberg departed Plymouth on Wednesday for her trip to New York to attend a United Nations climate change conference

Greta Thunberg, pictured with her father Svante, centre and skipper Boris Hermann, right, is sailing across the Atlantic in a racing yacht which uses renewable energy to generate its own electricity. Her trip to a UN climate conference will emit zero emissions

She is also planning to visit Canada and Mexico before travelling to this year's UN climate conference, which is taking place in Santiago, Chile, in December, making her journeys by train and bus.

The two-week sailing trip means she can attend the summits without using planes or cruise ships which cause greenhouse gas emissions.

She said her adventure would have challenges including seasickness but said many people in the world were suffering a lot more than that.

To keep herself occupied during the journey she has books, board games and a rabbit teddy bear, which was a gift from a friend.

The journey takes about two weeks - the yacht can travel at speeds of around 43mph but will be heading into the wind for much of the time so will be slower, and the captain wants a smooth ride.

The 60ft boat, Malizia II is capable of travelling at speeds of up to 43mph - powered by the wind - although it will be sailing mostly into the wind on its way across the Atlantic

Before setting sail, Herrmann said: 'The objective is to arrive safe and sound in New York.'

The yacht is made for racing, with foils, or wings, that lift it out of the water for a faster and smoother ride.

Inside it is sparse, fitted with high-tech navigation equipment, an on-board ocean laboratory to monitor CO2 levels in the water, and four bunks - Herrmann and Casiraghi will share one, sleeping in turns.

The toilet is a blue plastic bucket, complete with a biodegradable bag that can be thrown overboard, and meals will be freeze-dried packets of vegan food mixed with water heated on a tiny gas stove.

But state-of-the-art solar panels adorn the yacht's deck and sides while there are two hydro-generators, which together provide all the electricity they need on board.