Rabbi Tony Bayfield, the head of the Reform movement, this week stressed that the recent statement on pluralism signed with the Liberals and Masorti was not a declaration of war on Orthodoxy.
"We don't want to go to war with anybody," he told a quarterly meeting of the Reform's lay council in London on Sunday.
But explaining last month's statement in which Reform, Liberal and Masorti leaders pledged closer co-operation, Dr Bayfield said: "We do not think that an increasingly fundamentalist-orientated United Synagogue can meet the needs of 80 per cent of the community."
He added: "We are not waging war against fundamentalism or anything like that... We need this collaboration if this community is to have the bright future it should have."
At a discussion at the council, Reform members suggested co-operation over students, cheder education and youth camps.
Representatives of the three movements are due to meet shortly to consider how to implement the agreement in practice.
Stephen Moss, who took over as Reform chairman in summer, said: "We are not excluding anybody from engaging in the process."
He added that leaders of the three movements would be talking to their United Synagogue counterparts at a meeting in December of the community consultation committee - the inter-denominational forum set up as part of the Stanmore Accords of 1998.