By Caroline Wyatt
BBC defence correspondent
On board Ark Royal
The new vessels will be three times the size of Ark Royal
On a blustery flight deck, the top brass from the Royal Navy, RAF and industry gathered to mark the signing of the long-awaited contract for Britain's two new aircraft carriers. It is being held on board Ark Royal - one of the flagships they will replace.
If Ark Royal and Illustrious seem large, their successors will be a weighty three times the size, at 65,000 tonnes.
HMS Queen Elizabeth and HMS Prince of Wales will have flight-decks the equivalent of 49 tennis courts, and will be home to a crew of 1,450 Royal Navy and RAF personnel.
The First Sea Lord, Admiral Sir Jonathon Band, spoke of a proud day for the navy and for Britain, despite the years of debate, discussion and controversy over whether Britain can afford the two biggest warships ever to be built here.
"It has been a much talked about programme. It has emotion about it... big ships always do," he told the BBC after the signing ceremony.
"We are a maritime nation. This is a very exciting day for the navy - and this is a major contribution to Great Britain.
"It is exactly the sort of capability we should be investing in - and the navy is very proud to be its guardian."
The carriers will at first have to live without the joint strike fighter that is supposed to be on board - they won't be ready on time.
There has also been criticism that the carriers are too ambitious, and too much of a burden on an already stretched defence budget at a time when troops on operations in Afghanistan complain of a shortage of helicopters and other essentials.
Some suspect the go-ahead was given to the project because of its political appeal - guaranteeing some 10,000 ship-building jobs in key constituencies in England and Scotland.
Andrew Brookes, defence analyst at the International Institute for Strategic Studies (IISS), is among the critics.
"We can't afford this project - we can't afford the cost of the aircraft and the planes to go on them and ships to surround them. We cannot afford that without a major increase in defence funding, which I don't see coming."
Admiral Sir Jonathon Band and Baroness Ann Taylor signed the deal
Baroness Taylor, the minister for defence equipment and support, signed the contract on behalf of the government.
So how does she react to such criticism?
"The defence budget is under pressure," she admitted.
"We're spending more than we've ever spent, a real terms increase over last 10 years of 11%.
"But we wouldn't go ahead with the project if we couldn't afford it and we didn't think it was necessary."
Defence chiefs agree, saying that the carriers will offer Britain the ability to project firepower and influence across the globe, and help face future challenges and conflicts.
On board Ark Royal, Sub Lt Jonathan Orchard is one of the young naval officers looking forward to serving on the warships when they are built, in theory by 2014 and 2016.
"It is a fantastic idea really, and it secures jobs for the navy and the future of the navy's fleet air arm and surface flotilla.
"It is a lot of money no matter where you're coming from but people need to know it is worthwhile and the navy will use them well."
Royal Marine Duncan Hardy agrees that the Armed Forces have to look ahead and be prepared for future conflicts in other places.
"Aircraft carriers are great news - not just as a platform for the navy. It's UK defence strategy as a whole.
"Whilst we're currently busy fighting on two fronts in Afghanistan and Iraq, which is today's issue, we've also got to think about tomorrow."
The Conservatives, too, back the project, meaning the carriers - and the related jobs - should be safe, even if there is a change of government at the next election.
And as Ark Royal sails out of Portsmouth now, it is clear there is a real sense of pride within the Royal Navy that this maritime nation should soon be the guardians of what defence chiefs insist is a vital strategic investment for Britain's future.