The controversial Conservative peer Lord Ashcroft is to donate up to £5m towards a new royal yacht to mark the Queen's Diamond Jubilee.
The former Conservative Party deputy chairman has said he will contribute the money to the charitable trust trying to raise £100m to build a national flagship.
Retired Rear Admiral David Bawtree, the chairman of the trust, said he hoped Lord Ashcroft's move would encourage others to come forward and donate.
In a statement on his website, Lord Ashcroft said: "The murmurings this week of a privately financed replacement for Britannia are hugely heartening, even if there is a very long way to go to make this happen.
"But I would like to be one of the first to offer tangible support to the concept, by offering up to £5m towards the construction costs of building a new, effective and flexible royal yacht."
The proposed vessel – 650ft 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.
Lord Ashcroft added that the new vessel could be used by other members of the monarchy.
The Prime Minister has offered his support to the project, and in a letter to the rear admiral in October described it as a "splendid idea".
David Cameron's official spokesman said this week that he would "react favourably" to requests for government assistance but believed it would be "inappropriate" for taxpayers' money to be used.Reuse content