Boris Johnson's vision of a new national flagship to replace the royal yacht looks like a "1950s fishing trawler", according to a leading naval architect.
The prime minister announced last month that the £200m vessel would "represent and promote the best of British" around the world for the next 30 years.
But Stephen Payne, the designer of the massive ocean liner Queen Mary 2, was far from impressed by the artist's impression issued by Downing Street.
He told the Daily Telegraph that it would make a "very poor" flagship for the UK and would be more suitable for the Isle of Sark, population 500.
"I just think we could do something more ambitious," he added. "The superstructure front, akin to a 1950s Hull trawler, is great for a fair-weather ship but not such a good idea for a global voyager crossing the Atlantic, Pacific, or even rounding the tip of Africa."
He said: "They say they'll use a Royal Navy crew. Isn't there a chronic manpower shortage within the service? Will the Navy look at this new vessel not with adoring eyes but with despair as it struggles to keep frontline ships at sea?
"As for financing this ship, there's £200 million to find and I'd be surprised if the running costs weren't £5 million a year."
Mr Payne said he had sent Downing Street an outline of his proposals for a new royal yacht "Britannia 2" only for them to be lost.
He envisaged a 475ft ship with a 250-seat auditorium, an on-board pub, restaurant, TV studio, museum and souvenir shop.
"Britannia's importance stemmed from her royal status," Mr Payne said. "As a royal yacht, Britannia had an unambiguous cachet. Apart from her elegantly designed spaces, she had the Band of the Royal Marines and a hand-picked crew of more than 270. This package was what gave Britannia her standing, her prestige and the distinction which lured business on board."
The original Britannia was decommissioned by Tony Blair's Labour Government in 1997.
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