"Caution" has been urged on new figures suggesting that the number of workers on zero-hours contracts has increased threefold since 2010.
Labour said a newly-revised estimate, based on official figures, showed there were 583,000 people employed on zero-hours contracts, up from 183,000 in 2010.
But the Office for National Statistics (ONS) said in a statement that it urged people to treat the latest estimate with due caution, pending the forthcoming publication of "more reliable" figures.
The first employer-based estimate of the number of workers on zero-hours contracts is due to be published by ONS next month.
"ONS believes employers are best placed to provide accurate information about the employment terms of their workforce, and therefore in summer 2013 took steps promptly to begin collecting these data in order to produce better statistics on this important issue," said the statement.
Labour has pledged to outlaw the "exploitative use" of zero-hours contracts, under which employees do not know if they have work from one week to the next.
Shadow business secretary Chuka Umunna said: "Labour would tighten up the rules to outlaw zero-hours contracts where they exploit people and turn around the rising tide of insecurity we've seen under the Tory-led Government."
Some estimates have put the number of zero-hours workers at one million, prompting campaigns by trade unions to tackle the issue.
Ian Brinkley, chief economist at The Work Foundation, said: "The high levels of publicity in recent months mean that many more people may be reporting being on zero-hour contracts in 2013 compared with 2012.
"There is also no reason to think employers have dramatically increased their use of zero-hours contracts in the way suggested by the figures. The largely adverse criticism over the past 12 months of zero-hours contracts may have discouraged some employers who would have otherwise have used them and we know that some employers have said they will review their use of zero-hours contracts.
"The ONS is in the process of revising its methodology for calculating the number of people on zero-hour contracts. This should give a more accurate figure, because as the recent spike suggests there are people working on zero-hour contracts who do not recognise the term, and so when surveyed, do not report being on such a contract."
Unite union assistant general secretary Steve Turner said: "We fear that this latest estimate is just the tip of the iceberg of a zero-hour culture that is being exploited by bad bosses while the Government turns a blind eye.
"Our research shows that nearly three-quarters of people would not be on a zero-hour contract if they had a choice and a similar number are anxious about being on one.
"Zero-hour contracts are leading to stress and anxiety for ordinary people who are unable to plan from week to next or know whether they will be able to put food on the table.
"We need action now to end the exploitation and the insecurity of zero-hour contracts."