This will make them more than three times the size of the existing Invincible-class carriers.
Each 65,000-tonne vessel will be crewed by 1,450 sailors and airmen.
The building of the two aircraft carriers is expected to create or secure a total of 10,000 jobs across the UK.
The project is going ahead despite serious misgivings among some in the military about the huge financial burden it will place on a defence budget already under severe pressure.
The Army and RAF are already expecting cuts to equipment funds and defence analyst Andrew Brookes told the BBC that Britain could not afford the contracts.
"We can't afford the cost of the aircraft carriers, the cost of the Joint Strike fighters to go on them, and all the replenishment, escort and protecting vessels," he said.
"We can't afford that without a major increase in funding which I can't see coming."
They'll be able to pack a bigger punch, whether it be on a humanitarian operation or whether it be a full-blown strike operation
Adm Sir Jonathon Bond First Sea Lord
But Defence Secretary Des Browne said the carriers were "an affordable expenditure" and were not being purchased at the expense of other areas.
"The two aircraft carriers will provide our forces with the world-class capabilities they will need over the coming decades," he said.
"They will support peace-keeping and conflict prevention, as well as our strategic operational priorities."
Speaking in Govan, Mr Browne said the carriers would provide "very large floating bases for the Navy and the RAF", entirely under "sovereign control".
"They will allow us to project force," he said. "But they will also allow us to make a contribution to the protection of the sea lanes of the world, because as a trading nation we rely on those being secure."
Although both will eventually carry the new Joint Strike Fighter aircraft, those will not be ready on time for when the ships enter service.
This means that at first, the ships will carry the ageing Harrier aircraft instead.
MoD video showing how the new aircraft carriers will look
The First Sea Lord, Adm Sir Jonathon Band, told the BBC: "The reason for the size is that we've determined that we need to be able to put a weight of airpower on them from strike aeroplanes, and that has therefore determined their tonnage and their size.
"Basically, they'll be able to pack a bigger punch, whether it be on a humanitarian operation or whether it be a full-blown strike operation."
Construction of the ships' hulls will be split between Govan, Portsmouth and Barrow in Furness. Construction of the bow sections and the final assembly of all the parts will take place in Rosyth.
If it wasn't for this then the dockyard would be on the rundown to closure
Raymond Duguid, Rosyth dockyard
A further 145 jobs will be needed at BAE Systems at Frimley, Surrey and 250 at Thales UK in Bristol and Crawley to provide design, engineering and missions systems for the vessels.
Many more jobs will be created or secured elsewhere in related industries, and contracts have already been awarded to supply diesel generators, electronics and the steel for the ships.
Chairman of the Rosyth shipyard industrial joint council Raymond Duguid: said "If it wasn't for this then the dockyard would be on the rundown to closure, it would be that stark."
BBC defence correspondent Caroline Wyatt said the Navy sees the new carriers as its future flagships to replace Ark Royal and Illustrious, transforming Britain's ability to operate in hostile waters.
Janet Lowrie's son is currently serving in Iraq, but she is part of an organisation called Military Families Against The War. She told the BBC the money would be better spent on frontline equipment.
"Our vehicles are not worthy, they break down all the time, so replace them - save lives," she said.
HMS QUEEN ELIZABETH AND HMS PRINCE OF WALES
Displacement: 65,000 tonnes
Length: 280m (920ft)
Width (at flight-deck level): 70m (230ft)
Keel to masthead: 56m (184ft)
Nine decks (plus flight deck)
Speed: 25+ knots
Range: 8,000-10,000 miles
Aircraft: 36 F-35 Joint Strike Fighters and four Airborne Early Warning aircraft, plus EH 101 Merlin helicopters
Crew: 1,450 (including air crew)
Weapons: Phalanx close-in weapon systems; 30mm and mini-guns
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