Ministers should not reduce the conditions under which the UK would launch a nuclear strike, MPs have said.
The operational end of Trident's life is due to be 2024
A report from the Commons defence committee warned against "lowering the nuclear threshold".
MPs are due to debate government plans to replace Britain's Trident missile system - estimated to cost between £15bn and £20bn - next week.
The Ministry of Defence said it would consider using nuclear arms only "in extreme circumstances of self defence".
The select committee questioned the circumstances in which the government would use nuclear weapons after hearing evidence from Defence Secretary Des Browne.
He told MPs he could not be more specific about the reasons for launching a strike, saying "keeping the enemy guessing" was a key consideration.
In its report, the committee said: "Although we understand the need for ambiguity, the government should be clearer that this ambiguity does not lead to a lowering of the nuclear threshold."
Responding to the report, Mr Browne said it proved the government was right in seeking a decision to replace Trident now.
"Those who continue to argue for delaying this important decision should realise their approach would result in a policy of unilateral disarmament," he said.
The committee also questioned whether the government's pledge to cut the number of nuclear warheads by 20% had any "operational significance."
TRIDENT MISSILE SYSTEM
Missile length: 44ft (13m)
Weight: 130,000lb (58,500kg)
Diameter: 74 inches (1.9m)
Range: More than 4,600 miles (7,400km)
Power plant: Three stage solid propellant rocket
Cost: £16.8m ($29.1m) per missile
Source: Federation of American Scientists
While CND welcomed the report, it said replacing Trident would risk provoking a new arms race.
A spokesman said: "To use or threaten to use nuclear weapons is, under virtually every conceivable circumstance, illegal under humanitarian law.
"These are all issues that require widespread discussion and cannot be fully and adequately addressed in the seven days between the publication of the report and the debate and vote in Parliament on 14 March.
"Furthermore, the report issues a number of specific instructions and questions for the government, which also need to be addressed before the vote."