MPs have begun their inquiry into the UK's nuclear weapons arsenal.
Trident will be decommissioned by about 2020
The Commons defence committee is examining whether the UK should replace its Trident weapons system, which is expected to be obsolete by 2020.
It is taking evidence from a range of experts about the type of threat the UK might face in 20 years' time.
Tony Blair has pledged the "fullest debate" before any decision is taken. Critics say there is no threat which justifies replacing Trident.
Dan Plesch, from the Foreign Policy Centre, told the MPs' first hearing the UK would probably not be able to use nuclear weapons without America's agreement.
"The public understanding is that we have this if ever again we face 1940," said Mr Plesch, referring to the UK going to war with Germany without US backing.
"There's a strong sense, going back almost to the Armada and Trafalgar in our culture that we have to have some contingency."
People did not understand that with Trident, "the US would have every ability in the short and particularly in the longer term to prevent the system from being used because of our relationship", he said.
Sir Michael Quinlan, from the International Institute of Strategic Studies, said nuclear-armed terrorists were a "possibility" and a "pretty horrific" thought.
He said: "As far as deterring a state from using nuclear and other weapons of mass destruction, there's clearly an option there."
But Lee Willett, of the Royal United Services Institute, said it was impossible to "predict what will be around the corner" in 20 years or so.
Kate Hudson, chair of the Campaign for Nuclear Disarmament, said the UK no longer faced a threat from any nuclear-armed nation.
She said Mr Blair had said nuclear weapons were no use against terrorism, so Britain "should be positioning itself as a global leader for peace by ending its nuclear weapons programme".
'Window of opportunity'
After the hearing, she added: "We should take this window of opportunity to begin disarmament discussions as required under the nuclear non-proliferation treaty."
Foreign Secretary Jack Straw says the UK is "entitled" to have nuclear weapons.
Mr Blair said a decision on replacing Trident would be taken before the next election. By the end of the year "we should have a clear idea of the timeline" for a debate on its replacement, he said.
However, he would not commit himself to a Commons vote on replacing Trident.
For the Conservatives, shadow defence secretary, Dr Liam Fox, said he believed a question remained over the Government's willingness to replace Trident.
He said: "We are committed not only to retaining the current nuclear deterrent but also to replacing it when necessary, and remain to be convinced that there is a delivery system better than a submarine based system."
The MPs on the Defence Committee do not have the power to decide on policy, but they are influential.
The committee is holding a series of inquiries over the course of the current Parliament, "with the intention of informing the public debate on the future of the UK's strategic nuclear deterrent".
It will consider the timetable in which decisions on replacing Trident will have to be taken and implemented.
The cost of replacing the UK's four submarines armed with Trident missiles could reach £20bn according to some estimates.