Membership of the Campaign for Nuclear Disarmament (CND) has increased dramatically since the war with Iraq, the group said as its annual conference started in Birmingham.
CND helped organise February's huge ant-war rally in London
The organisation said it had been struggling to cope with a flood of applications in the wake of the huge protests earlier in the year against the UK's involvement in military action.
CND helped organise the massive demonstration in London in February, when between 500,000 and two million people marched through the capital before the bombing of Iraq started.
Young people in particular were keen to join the peace movement and protest against nuclear weapons and armed conflict in general, CND said.
The organisation has been calling for an independent inquiry to find out "who, when and
why" the decision was taken to take part in the war.
CND chair, Carol Naughton, said: "It is apparent through all the lies and duplicity that the government was
hell bent on going to war with Iraq.
"CND exposed the immorality and illegality of this war, causing the
government to spin even harder to persuade the public and parliament to support them.
Carol Naughton accuses the government of "lies and duplicity"
"The whole episode is a tragedy and must never be allowed to happen
The Iraq conflict is among the subjects for debate during CND's annual conference, which is being held at Birmingham University this weekend.
CND membership topped 100,000 in the 1980s, when cruise missiles arrived in
the UK, but fell below 30,000 two years ago.
Membership is now said to be well over 32,000.