Former Liberal Democrat leader Sir Menzies Campbell has said there is "an element of rush" about the proposed timetable for the government's strategic defence and security review (SDSR).
"To set a time limit of a few months when you consider the complexity of the issues at stake is unwise," he told MPs in a debate on the review on 21 June 2010.
A large part was going to be "an expenditure review and not necessarily a defence review", he argued.
On Trident, Sir Menzies said: "I don't see how you can have a value-for-money assessment unless you consider what other alternatives are available.
"In that sense the review that the coalition document embraces and endorses is going to be much wider than many people think."
But Tory backbencher Bernard Jenkin insisted Trident was the cheapest and most effective option.
"Its costs are vastly over-stated by its opponents, amounting only to 0.5% of the defence budget over its lifetime," he told MPs.
Labour's John Woodcock added: "Unilaterally scrapping or delaying the renewal of Trident would make our country and the world less safe, not more.
"Instead it is vital that we secure genuine progress on the multilateral non-proliferation talks currently under way," he argued.
"We must guard against the dangerous spread of woolly thinking on this issue."
Watch the first part of the debate