Tony Blair told a press conference in London that unless the Tories managed to take control of a number of new local authorities, then they were in severe difficulty.
But he then added: "Of course we don't want serious losses at all. What I'm saying is that we had in 1994 an extraordinarily good set of results.
"It would defy all the rules of politics if we managed to achieve as good a result or anything near as good a result as that, although the feedback on the ground is actually good, but there is the issue of turnout."
In a considered appeal to people who had previously voted Labour, the Prime Minister said: "I know that many Labour voters believe they did their job last year by voting for the Labour government. It would be a real mistake to stay at home on Thursday and not bother to vote.
"That way people could end up getting a Conservative or Liberal Democrat council by the back door. To get the best deal for local people, it's important that local councils and central government work together."
That appeal - identical to appeals made by the Tories when they were in office - underlined the fears of the party machine that they could be facing embarrassing losses and the appearance of a loss of momentum.
But Mr Blair added that the Tories had already claimed they were on track for a political comeback in the elections, and he said: "They claim to have reformed their party, they claim to have learned the lessons of defeat.
"If they don't make very big gains this Thursday it will be a crushing blow to them."
Paul Burstow, the Liberal Democrat local government spokesman, said yesterday that the Conservatives needed to make 500 gains nationwide, and the Liberal Democrat leader, Paddy Ashdown, said his party offered relief from one- party rule - under whichever party.
Asked about Labour's spending review, Mr Blair said that it would generate extra investment for priority areas like schools and hospitals, but he warned that extra cash would be tied to reform.
"We want better schools, better hospitals," he said.
But he added: "That comes not just through more money but by making sure also that we have the right structures in place and the right programme in place for reform and change."
More than 4,000 seats are being contested in the elections, including all those on the 32 London boroughs.Reuse content