PLANS to build the highly-anticipated “world’s biggest superyacht” could be ditched after the millionaire tycoon behind the scheme failed to pay up.

The 728ft-long vessel, Somnio, has an eye-watering £480million price tag, and the British designers are suing the businessman who is supposed to bankroll it.

Australian Car Le Souef, 62, is the minted professional investor who came up with the mind-boggling boat plans back in 2007.

He envision it as an “unprecedented feat of design and engineering […] set to change the course of ultra-luxury travel forever”.

The uber-luxury ship was dreamt up as a combo between a “six-star hotel” and a private yacht.

It was to be a floating city for the super-rich, with each of the 39 plush cabins sold off to different owners – with a cool £8.1million starting price.

British private jet and superyacht specialists Winch Design picked up the plans and began working on the project in 2019, which it was “very excited” about.

The high-end apartments were meant to go on sale last year, to much fanfare, in the “most exclusive address in the world”.

But 2024 has been and gone – and still the yacht is yet to hit the water.

The sticking point is a whopping £734,000 bill that Le Souef has failed to pay – for work carried out in 2021 – and that Winch has now successfully sued him for.

Judge Lesley Anderson KC said at London’s High Court: “This is a dispute about an ambitious project […] to build a luxury yacht, comprising several single, luxurious, residential apartments, which is intended to be one of the largest and most luxurious yachts ever to be built.”

He added that there was “some disagreement” over Le Souef’s true net worth and how the project was pitched to Winch.

The judge said: “According to Mr Le Souef, the total estimated cost of the project is $500 million.

“According to its accounts for the year ended 30 June 2021, Somnio had raised $760,000 for the project’s initial research and feasibility costs.”

The judge said that Winch had signed a contract with Le Souef in 2021 when he agreed to pay £5.25million for design work in instalments.

He coughed up the first £200,000, but the next three payments after that never appeared – forcing Winch to stop working on the project.

Four years later, after still no payment, the designers have sued Le Souef for its money.

The Australian battled the case on various grounds, claiming that he was not personally responsible for the payment and that a senior Winch figure had agreed not to sue.

He agreed that Winch had the right to stop working on the boat as they did.

However, the judge dismissed Le Souef’s counterclaim and ordered him to pay Winch £733,750.

Judge Anderson KC said that Mr Le Souef, was “surprisingly defensive” in evidence.

She said: “Despite being an experienced businessman, he initially claimed not to have come across contracts although he shortly qualified that by saying he meant contracts relating to the general operations of the business which did not involve intellectual law.”