A group of former defence secretaries and military chiefs have voiced their support for a £25 billion replacement of Britain's nuclear deterrent system, ahead of a long-awaited report which is expected to argue the case for a scaled-down Trident force.

Liberal Democrat Cabinet minister Danny Alexander's Trident Alternatives Review is expected to include an array of options short of like-for-like replacement, including a proposal that the Royal Navy's fleet of nuclear-armed submarines could be cut from four to two.

The review of alternatives results from a compromise reached by Conservatives and Liberal Democrats in the 2010 coalition agreement which brought them into government together, and is likely to set a clear dividing line between the parties at the next general election.

Treasury Chief Secretary Mr Alexander last week said he hoped today's review would "open up a debate about the fact that there are different ways of approaching nuclear deterrents that are responsible with our nation's security whilst recognising that just like other aspects of our defence we don't have to be stuck in the same Cold War postures of the past forever".

The fleet of four Vanguard-class submarines which carry the Trident missiles are due to be replaced from 2028, with a final decision on whether to replace all four boats due to be taken in 2016 - after the 2015 election.

A leaked version of Mr Alexander's review suggested that building just two new subs instead of four would save £5 billion in upfront expense and a further £1billion in running costs. Other options thought to be put forward in his report include a proposal to cut costs by sometimes putting subs to sea without warheads on board. However, unilateral nuclear disarmament will not be included as an option to be considered.

But Defence Secretary Philip Hammond yesterday warned it would be "reckless" to downgrade Trident, insisting that two subs would not be able to offer the "continuous at-sea deterrence" which Britain has maintained since the 1960s.

And a letter to the Daily Telegraph, signed by five of Mr Hammond's predecessors as defence secretary and two former chiefs of defence staff, warned the Government not to "take risks with our security" by downgrading Trident.

"In an uncertain world, in which the number of nuclear weapons remains high and some states are increasing their holding, we should not take risks with our security by downgrading to a part-time deterrent," they wrote.

"We cannot possibly foresee what threats will develop over the next 30 years. Reducing our submarine-based Trident capability would weaken our national security for the sake of a very small fraction of the defence budget.

"It is our view that if Britain is to remain a leading global power with strong defences, nothing less than a continuous-at-sea deterrent will do."

The letter was signed by former Conservative defence secretaries Liam Fox and Sir Malcolm Rifkind, alongside Labour's former defence secretaries Bob Ainsworth, Lord Reid of Cardowan and Lord Robertson - who was also the secretary general of Nato between 1999 and 2004. Also signing were ex-chiefs of defence staff Lord Boyce and Lord Stirrup.

Prime Minister David Cameron has left no doubt of his preference for like-for-like replacement of the ageing Trident fleet, which was also backed by Labour's Tony Blair in a 2006 review when he was Prime Minister.

Labour defence spokesman Kevan Jones said: "In 2007 Parliament took the view - supported by the Labour Government of the day - that a continuous submarine-based system with ballistic missiles provided for the minimum credible nuclear deterrent, and was the most-effective model to meet our strategic needs.

"Labour is committed to examining any new evidence rigorously in order to establish whether there are alternatives to the conclusions of the last review in 2006.

"That examination must feature two priorities - capability and cost. They must be our guiding principles. Labour wants the UK to have the minimum credible deterrent, in line with our national security needs and our international obligations, and we want to ensure that we achieve maximum value for money. We look forward to close examination of the Government's review of alternatives."

A new analysis of polling on the nuclear issue found that public opinion has shifted away from full-scale replacement of Trident since the 2006 review, and suggested that opting for a lower-cost option may not be a vote-loser in 2015.

The report from the British American Security Information Council found: "Polls suggest that opinion has moved towards relinquishing nuclear weapons after Trident when given a simple yes/no choice. This is generally strengthened when respondents are given a cost of £20-25 billion for the capital costs of replacing Trident starting with a new fleet of ballistic missile submarines.

"Opinion is split more evenly three ways when a third option of a smaller, cheaper replacement is introduced. Data, here, suggest the electorate is broadly in favour of keeping nuclear weapons in some form, but against a like-for-like replacement of the current system."

The report found that the issue of nuclear weapons has "relatively low salience" among UK voters, adding: "The indeterminacy of public opinion gives all three main Westminster parties political space to rethink UK nuclear weapons policy after Trident or recommit to current policy. Polls suggest the electoral consequences of policy change or stasis are unlikely to be decisive."

Meanwhile, the Campaign for Nuclear Disarmament released its own report, entitled The Real Alternative: What the Government's Trident Alternatives Review Isn't Telling You, which branded the omission of unilateral disarmament from Mr Alexander's document an "abdication of responsibility".

Non-replacement of Trident is "a credible option, which offers serious strategic and economic benefits", argued CND, citing increased budgetary flexibility for the Ministry of Defence, a strengthened non-proliferation regime, economic savings of more than £100 billion over the lifetime of a successor system and a moral and diplomatic leadership position for the UK.

A Conservative source said: "We expect that the long-held Lib Dem claim that there are cheaper alternatives to Trident will be exposed as a myth.

"It is widely anticipated that that Lib Dem cruise missile-based system will be seen as too expensive and not as credible as the Trident system, so they are having to backtrack and change their plans by punting for a part-time deterrent instead.

"To date, there is no evidence to say that building just two new submarines instead of the four needed to maintain a continuous-at-sea deterrent would save £5 billion in upfront expense and a further £1 billion in running costs. These are figures made up on the back of a fag packet. "