A group of former defence secretaries and military chiefs have voiced their support for a £25 billion replacement of Britain's nuclear deterrent system, on the eve 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 his 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 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.