A new generation of nuclear weapons is expected to be backed in a government white paper to be published on Monday.
Trident is due to become obsolete by 2024
Ministers will outline their preferred option for Britain's nuclear arms capability. The lifespan of the current Trident missile system ends in 2024.
The Observer reported that Tony Blair would pledge to consider reducing both submarines and warheads, in a gesture to Labour's nuclear critics.
The white paper will be followed by a three-month consultation and MPs' vote.
Ministers say a decision is needed now on Trident's future, to ensure any replacement is ready by 2024.
Prime Minister Tony Blair and Chancellor Gordon Brown, widely tipped as his successor, have both indicated they support replacing Trident.
Mr Blair has said the system - 64 missiles based on four nuclear submarines - is an essential part of Britain's ability to defend itself.
TRIDENT MISSILE SYSTEM
Missile length: 44ft (13m)
Weight: 130,000lb (58,500kg)
Diameter: 74 inches (1.9m)
Range: More than 4,600 miles (7,400km)
Power plant: Three stage solid propellant rocket
Cost: £16.8m ($29.1m) per missile
Source: Federation of American Scientists
The Observer on Sunday reported that Mr Blair would pledge to consider scaling down the submarine fleet to three and to reduce the number of nuclear warheads - to appease opponents of nuclear weapons within the Labour Party.
But former defence minister Peter Kilfoyle told the BBC it was a "very inopportune moment" to replace Trident.
"Politically, it is extremely sensitive," he told BBC Radio 4's The World This Weekend.
"We need a full and protracted debate. We don't need to rush to some kind of decision over the next few months as appears to be the government's wish."
The white paper will give the various options and why they are considered acceptable or not, and MPs will vote on a single recommendation.
Options could include whether to keep a submarine-based system, change to a land-based or aircraft-based system, or possibly to overhaul, rather than replace, the submarine fleet to extend its lifespan further.
Supporters argue Trident is needed to deter any threat - particularly at a time when countries like North Korea and Iran harbour their own nuclear ambitions.
Former head of the Royal Navy Admiral Sir Alan West said it would be foolhardy to give up the deterrent of nuclear weapons in an "extremely dangerous" world.
"The messages we would give, I think, by giving it up would be very strange," he told the BBC.
But critics say the estimated £10-25bn cost would be better spent elsewhere and Trident was designed for the Cold War, not modern threats of international terrorism.
The Conservatives support retaining nuclear weapons while the Liberal Democrats have said the number of nuclear warheads should be halved to 100.
Anti-nuclear campaigners say they fear the government has already decided to go ahead with replacing Trident.