A new national flagship yacht, designed to promote the UK around the globe, will cost up to £250m, the defence secretary has said.
A government document published last week put the budget at £150m, while reports said it could reach £200m.
But Ben Wallace has now given "a firm price" for the "floating embassy" of between £200m and £250m.
Labour's shadow home secretary, Nick Thomas-Symonds, said the party would scrap the government's "pet project".
Boris Johnson announced the plan for the yacht in May, which will be used for hosting events like trade fairs and diplomatic meetings.
The vessel, which is yet to have a name, will be the successor to the Royal Yacht Britannia, which was retired in 1997 after 44 years in service.
It will be funded from the Ministry of Defence's budget.
The prime minister said the ship would reflect "the UK's burgeoning status as a great, independent maritime trading nation" post-Brexit.
But Mr Thomas-Symonds said the latest costing showed the PM had "lost control of his vanity yacht", calling for the money to be invested "wisely" on projects such as tackling crime and anti-social behaviour.
An "engagement day" to discuss the new flagship was held in Greenwich, London, on Wednesday, with Mr Wallace giving a speech.
He said there had been "a lot of reporting around this ship, not all of it accurate" so wanted to set out the government's "basic aims" for the vessel.
The defence secretary added: "Subject to working through bids, competition and technology, I aim to commission the ship for between £200m and £250m on a firm price.
"The competition will run until the end of October. I hope to announce the winners in December."
After that, Mr Wallace said he wanted construction to begin "in a British shipyard as early as next year and have a ship in the water by 2024 or 2025".
He added: "That's an ambitious timescale but this is an ambitious project, the chance to break the mould and break some records to get things done in the national interest."
Mr Wallace previously defended the cost of the ship to the MoD, telling the Commons Defence Committee it was less than 0.1% of the £13bn defence budget for shipbuilding over the next 10 years.
But Labour's shadow Treasury minister, Bridget Phillipson, called on the government to show "a real focus on value for money at every stage" of the project and ensure it supported jobs in UK shipyards.