Prime Minister David Cameron has offered his support to a project to create a new Royal Yacht to mark the Queen's Diamond Jubilee, but has ruled out spending public money on the vessel.
The chairman of a charitable trust which is trying to raise £100 million to build a new national flagship revealed that the PM has written to him to offer his "full support" to the initiative.
Mr Cameron's official spokesman said that the PM would "react favourably" to requests for Government assistance, but believed it would be "inappropriate" for taxpayers' cash to be used on the project.
Retired Rear Admiral David Bawtree said he approached universities minister David Willetts with his proposals for a Future Ship Project for the 21st Century last year.
The proposed sailing ship - 650 feet long and as tall as St Paul's Cathedral - would provide education and vocational training for young people, facilities for scientific research and a venue for trade missions and commercial exhibitions as well as staterooms for the use of the Royal Family on trips around the United Kingdom and overseas.
Mr Willetts backed the project in a letter to the PM last September, as did Education Secretary Michael Gove.
And Mr Cameron wrote to Rear Admiral Bawtree in October welcoming his "truly inspiring initiative".
The PM said that he was keen on projects that aim to help young people from all backgrounds to develop the skills and energy that Britain needs for its future prosperity, said the Rear Admiral.
His letter said: "I think this is a splendid idea and you have my full support. I know you intend the project to be privately-funded and there is some way to go, but you understand that the Government is not able to offer any financial assistance."
Rear Admiral Bawtree, the former commander of Portsmouth's naval base, said he was not seeking state funding, but that Mr Cameron's letter would boost efforts to secure private donations.