Tony Blair has warned Labour activists against moving the party to the left to halt falling membership.
Tony Blair warned that leftist policies would lose Labour power
The prime minister said his public service reforms had allowed Labour to keep in touch with the public mood.
Speaking at the Progress conference in London, he warned that shifting to the leftist policies of the past would see the party lose power.
"We know where that leads: to a right-wing Conservative government," he said. "Always has done, always will."
Labour membership has fallen to below 200,000 - its lowest level since Mr Blair became leader in 1994.
Mr Blair's critics have claimed that the fall in membership has been driven by reforms such as foundation hospitals and university top-up fees.
But the prime minister was defiant in his speech on Saturday, saying that the party's success was underlined by Tory leadership contenders "scrambling" to copy its policies.
Labour should continue to claim the "centre ground" in politics, he said.
"What we must not do is fall for some modern version of the old left delusion that the problem with the progressive government is that it is not left enough, and if only its leadership rediscovered its true principles all would be well."
Urging activists to remain focused on retaining power, he added that the party had to be organised as a "broad-based political movement" to keep in touch with voters' needs.
"Because to be in touch is to be in power and even if not in government, always to be a contender for it," he said.
Over the next year, Mr Blair said the leadership would consult with members to change the party's organisation so that it connected better with its supporters.
But Tony Woodley, general secretary of the Transport and General Workers' Union, said: "It is strange that party leaders only want to 'revive or modernise' our relationship after suffering heavy defeats at conference."
He added: "It is time to wake up and smell the coffee on issues that matter most to working people.
"Failing to do so goes some way to explain why party membership is falling below 200,000."
Progress is an independent organisation for Labour members and trade unionists.
Its honorary president is Alan Milburn, the former health secretary who was in charge of Labour's campaign for the general election in May.
Mr Blair's comments echo the defence of public sector reforms he made at the Labour Party conference last month when he claimed he wished he had been more radical.
He said: "Every time I've ever introduced a reform in government, I wish in retrospect I had gone further."