John Reid has said it would be "nothing short of mad" for Labour to abandon the centre ground of British politics.
John Reid said the party had 'moved on'
The defence secretary warned the party should never go back to a "high tax, irresponsible spending, state control, life dictating old Labour Party".
Peter Mandelson told the same conference fringe meeting the party must remain "true to New Labour".
Labour's conference is likely to be dominated by debates about the party's direction after Tony Blair's departure.
Many on the left hope a party led by Gordon Brown, the man widely expected to take over from Mr Blair when he retires, would be more left wing.
And union leaders have expressed doubts about the proposed extension of private sector involvement in the public services.
Peter Mandelson urged the party to "be true to New Labour"
But Mr Reid warned a meeting of Blairite think tank Progress the party must not "go back to where we were in the 1970s".
He said the party was formed to represent the interests of working people but had to adapt to a more individualistic age.
"Times have changed. People have moved on and we have moved on. That will be the message of this conference."
He said there were injustices still to tackle in health and education which needed radical solutions.
"Refusal to countenance radical change would be to leave those injustices embedded in the very system to we are trying to change," Mr Reid told the meeting.
His words were echoed by former health secretary and election co-ordinator Alan Milburn, current health secretary Patricia Hewitt,
Also speaking at the meeting were Culture Secretary Tessa Jowell, communities and local government minister David Lammy, former education secretary Stephen Twigg, who lost his Enfield North seat in this year's general election, and Peter Mandelson.
Mr Mandelson urged the party to "be true to New Labour" and to make a "social justice argument" for social and economic reforms.
He said the party had to be "vigilant about those who want to return to the old left/right dividing lines of the past."
And he said the "spirit of New Labour" must not be allowed to vanish "into thin air" in the "years ahead" when Tony Blair is not the party leader.
"The essence of New Labour is to respond to what is going on in the real world. The essence of New Labour is to respond to public concerns, rooted in our values but without dogma," he told the meeting.
He also urged the party not to abandon its traditional pro-European stance just because Europe needed to be modernised.