Campaigners have rounded on reports that the government is planning to replace the UK's ageing Trident submarine nuclear deterrent.
Trident will run for about another 20 years
Tony Blair said no decision had been taken, but he believed it was best that the UK retained the capability.
The Greens say it would be a step backwards and "immoral".
The Lib Dems say Parliament must have a full role in deciding Trident's future. The Tories say in principle they would keep an independent nuclear deterrent.
Caroline Lucas, of the Green Party, said if the newspaper reports were true it was an example of "breathtaking hypocrisy".
"I don't see what possible moral authority we have in lecturing other countries, like Iran, about not developing their nuclear capacity if we ourselves are going ahead and upgrading our own nuclear weapons.
"I think this is a real step backwards, it's a step that will lead to greater insecurity and I think it is an illegal and immoral step."
And Marian Hobbs, New Zealand's disarmament minister who is leading demands for existing western nuclear powers to disarm, told the BBC she wished she could persuade the UK not to replace Trident and to move away from nuclear weapons.
It follows a report in the Independent, denied by Mr Blair, that he had secretly approved a new generation of nuclear weapons to replace the nuclear-powered Trident missile submarines.
Mr Blair said on Monday that no decision had been taken, but it would be made at some stage "in the next few years".
"I think that it is best that Britain retains an independent nuclear deterrent but before you come to a final decision on that you have to look at all sorts of different aspects of it, including cost," he said.
The Trident fleet is not due to be decommissioned for about 20 years, after which it will either need replacing or upgrading if the UK is to keep an independent nuclear deterrent.
'Symbol of power'
The idea is that the UK has a "second strike" capability to deter an aggressor from making the first strike against it.
But critics say it is outdated, designed to deal with the threat posed by the Soviet Union during the Cold War, and should now be disbanded.
SNP leader Alex Salmond told BBC Radio 4's Today programme: "I think the vast majority of people are sceptical about the reason for spending a minimum of £20bn on a symbol of power and prestige.
"[It] does nothing to enhance the security of this country and does everything to divert resources away from things that actually matter, like the economy and public services."