Mr Hague's speech in full and the response from MPs: from BBC Democracy Live
The government has said it will be "more open" about the UK's nuclear weapons capability after announcing it will retain a maximum of 225 warheads.
It was revealed in 2006 the UK had 160 "operationally available" warheads but the extra numbers needed above this to allow for maintenance were not stated.
Foreign Secretary William Hague said the disclosure would help build trust between nuclear and non-nuclear states.
The US and France have already released details of their total warhead numbers.
The new coalition government has also said it will review the long-standing policy dictacting when the UK would be prepared to use nuclear weapons to make sure it is still up to date and realistic.
At the moment, the UK would only contemplate such a drastic move in self-defence - including the defence of Nato allies - and only in extreme circumstances.
By Paul Reynolds, the BBC News website's world affairs correspondent.
The announcement repeats what the then Prime Minister Tony Blair told Parliament in December 2006, that the UK has "no more than 160" operationally available warheads.
But it goes on to reveal that the total number of warheads will "not exceed" 225, a number slightly higher than some estimates.
The statement has been deliberately made during the nuclear non-proliferation treaty review conference in New York and is designed to demonstrate greater British transparency in line with France and the US which have previously revealed their stocks.
Of interest as well is the additional announcement that British nuclear weapons policy is to be reviewed and presumably refined.
At present it simply states that the UK would consider using nuclear weapons in self defence or in defence of a Nato ally.
The US recently stated that it would limit its nuclear responses and the UK appears to be coming into line with that.
The announcement comes as the UK and other signatories to the nuclear non-proliferation treaty are discussing global nuclear developments at a conference in New York, a meeting dominated by concerns over the international community's stand-off with Iran over its nuclear programme.
Disclosing the maximum warhead figure for the first time, Mr Hague told MPs - in a debate on the Queen's Speech - that the time was right to be "more open" about the number of weapons the UK retains.
"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," he said.
Additional warheads are needed to ensure the number of those ready to be deployed at any one time never falls below 160, taking into account routine maintenance and logistical management of the stockpile.
The US revealed earlier this month that it had 5,113 nuclear warheads while France said in 2008 that it had 300.
The BBC News website's world affairs correspondent Paul Reynolds said the 225 figure was "slightly higher" than some estimates.
The future of the UK's nuclear deterrent is regarded as one of the main points of tension between the Conservatives and the Liberal Democrats in their coalition arrangement.
The Lib Dems opposed the like-for-like replacement of the Trident missile weapons system but agreed to support the move in return for Conservative guarantees that the project would be closely scrutinised for value for money.
Also speaking in Parliament on Wednesday, former Lib Dem leader Sir Menzies Campbell said it was "illogical" not to consider the future of Trident within the strategic defence review, expected to begin later this year.
The UK's strategic basis for considering a nuclear attack has not fundamentally changed since it acquired nuclear capability in the early 1950s.
"The UK has long been clear that we would only consider using nuclear weapons in self-defence, including the defence of our Nato allies," Mr Hague added.
"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."
The US and Russia agreed large cuts to their nuclear stockpiles earlier this year and the UK has said it wants to play a "full part" in supporting non-proliferation and the reduction of weapons levels worldwide.
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