But the party's critics warned of a "massive but passive" membership which was being used as a fan club for a leadership that was increasingly tightening its control over a centralised machine.
Labour leaders have spoilt their achievement by overclaiming, but the increase of 54 per cent in the 30 months since Tony Blair became leader in July 1994 is a marked turnaround at a time when many assumed that the age of the mass political party was long past.
Mark Seddon, editor of the left-wing Tribune newspaper, compared the party's new recruits to members of the National Trust.
"The upsurge has been in all those places where people are more likely to respond to demands for money from the national party," he said, rather than contributing to more active local parties.
He said that an increasingly middle-class, inactive membership was part of the leadership's plans, revealed in The Independent this week, to abolish constituency-wide party committees and to end the policy-making role of the party conferences.
He promised all-out resistance: "I just don't think it's going to happen. The constituencies won't have it," he said. Mr Seddon added: "I'm delighted the membership is growing, but you have got to treat these numbers with a degree of scepticism."
He compared the Blair boom to the heyday of CND and the launch of the SDP, whose mass recruits failed to last. He added that there was plenty of evidence of people whose membership had lapsed when they became disillusioned, but whose names had continued to appear on membership lists.
A Liberal Democrat spokesman also cast doubt on the methods which the Labour Party had used to achieve an 85 per cent retention rate. He said that Terry Marsh, the former boxer and Liberal-Democrat candidate in Basildon, whose Labour membership lapsed two years ago, had received a ballot paper for October's vote on Labour's early manifesto.
Conservative Central Office estimates that there are 750,000 members of its autonomous local associations, based on a survey taken in 1995.
"While we accept that we have lost some members since then, we believe that overall we have gained substantial numbers as well," the party said in a statement yesterday.
Mr Prescott poured scorn on the figure, citing independent estimates as low as 300,000 as the basis of Labour's claim to have overtaken the Tories.
"I challenge them to produce a list of every single member of the Tory party," he said, and threw down the same gauntlet to the Liberal Democrats, who do have a national list and have claimed about 100,000 members over the past four years.
Mr Prescott launched Labour's recruitment effort at the end of his first party conference as deputy leader in October 1994 with one of his famous word-tangles. "Let's start talking and stop implementing this membership drive," he said.
Yesterday, he set a new target of 500,000 by the end of 1997 - which represents a minor slippage from the deadline of the general election which he set two years ago.
But the big unanswered question is: are the new members New Labour?
The party last year carried out an analysis of its members who pay the full pounds 16-a-year subscription. The survey revealed that they were more middle-class than the average member of the Tory party, but it did not reveal anything about their politics.
The last academic study of Labour members' attitudes, by Professor Patrick Seyd of Sheffield University, was carried out when John Smith was leader in 1992.
Professor Seyd admits the lack of data is frustrating: "Mr Blair obviously thinks they are malleable credit card payers, whereas the left thinks they are crypto-SDPers. We simply don't know."
The Independent conducted its own mini-survey of 50 new members in the Brighton Pavilion constituency in September, 1995, which suggested that their attitudes were similar to those of existing members. Many of the new joiners were lapsed members or long-time party supporters.
Last October's ballot for the national executive is the most recent indicator, and it suggested there might be a shift in favour of more "Blairite" candidates. The poll was overshadowed by the drama of whether Harriet Harman would hold her place in spite of sending her son to a selective grammar school, which she did, comfortably. That was itself significant.
The poll also saw another rise in the votes for the left-wingers Diane Abbott and Dennis Skinner, both of whom were up 9,000.
But there were bigger rises for Robin Cook (24,000), David Blunkett (18,000), Mo Mowlam (15,000) and Gordon Brown (14,000). The first two are regarded as "soft left", but Ms Mowlam and Mr Brown are unrepentant "modernisers".
Letters, page 11
Who belongs to who?
t Average age of Labour members: 42
t Average age of Tory members: 62
t One-third of Labour members who pay full subscriptions have annual household incomes over pounds 30,000
t Only one quarter of Tory members have annual household incomes over pounds 30,000
t Of full-fee Labour members, 86 per cent are home-owners
t The Tories claim 750,000 members, but independent estimates put the figure between 300,000 and 500,000
t The Liberal Democrats claim around 100,000 members
t The German Social Democrats have 800,000 members
t The Swedish Social Democrats have 260,000, equivalent to 1.7 million for a country the same size as the UK
t The French Socialist Party has only 135,000 members
t Labour claims an 85 per cent retention rate, with twice as many members joining as leaving
t There are two women for every three Labour men
t Tory membership peaked at 2.75 million in the 1950s
t Labour membership peaked at 1 million in the 1950sReuse content