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Describing what he called a "more open" policy, Hague said Britain's total number of nuclear warheads would not exceed 225, including the maximum 160 already declared as "operationally available".
He also signalled that the coalition government is likely to downgrade the importance of nuclear weapons in military strategy reflecting decisions announced last month by the US.
The British review is expected to conclude that the UK would rule out using nuclear weapons in retaliation against attacks involving biological or chemical, or conventional non-nuclear weapons.
However, it is expected to make an exception, as the Obama administration did, for Iran arguing that Tehran is covertly developing nuclear weapons.
Successive British governments have based their nuclear doctrine on the principle that potential enemies would be deterred by the principle of "calculated ambiguity" or "uncertainty" about the circumstances in which they would be launched. The Labour government suggested it might use them against an attack with biological or chemical weapon by a non-nuclear state.
"We believe that the time is now right to be more open about the weapons we hold. We judge that this will assist in building a climate of trust between nuclear and non-nuclear weapons states and contribute therefore to future efforts to reduce the number of nuclear weapons worldwide," Hague told the Commons.
He said: "The UK has long been clear that we would only consider using nuclear weapons in self-defence, including the defence of our Nato allies.
"However we are prepared to look again at our declaratory policy to ensure that it is fully appropriate to the political and security context in 2010 and beyond, and we will begin this work now."
Hague added: "This country has been deliberately ambiguous over the precise circumstances of use."
Liam Fox, the defence secretary, said: "Openness and transparency are essential if we are to move towards multilateral disarmament." The moves were announced as the Nuclear Proliferation Review conference in New York is drawing to an end.
The Labour government said four years ago when it announced a decision to renew the Trident missile system that it would cut Britain's 160 "operational warheads" by 20%. However, it had yet to do so, Whitehall officials said.
A decision has also to be made during this parliament about whether new warheads would be developed at the Aldermaston Atomic Weapons Establishment. The Obama administration said in its new Nuclear Posture Review last month that the US would not manufacture any new nuclear warheads.
Former Liberal Democrat leader Sir Menzies Campbell called for the Trident nuclear defence system to be included in the forthcoming strategic defence review- – something the Conservatives have so far opposed – saying it was "illogical" to exclude it.
Under the coalition agreement, Trident would only be examined on the basis of "value for money".
Campbell said: I don't believe you can consider it from the point of view of value for money, if you don't consider the requirement for it and whether there are reasonable alternatives."
France has said its arsenal will include fewer than 300 nuclear warheads.. The US announced earlier this month that it had 5,113 warheads in its stockpile.
Source: The Guardian