As many as 5.5 million Britons could be signed up to work contracts that promise them less than three hours of work a week, five times more than existing estimates, new figures suggest.

A survey of 5,000 members of Unite, Britain's biggest union representing more than 1 million people, found that 22% of workers employed by private businesses had deals that offered little or no guarantee of work and pay.

Across the entire UK workforce, the figures suggest millions could be employed on zero-hours contracts, which often provide no holiday or sick pay but can leave employees having to ask permission before seeking additional work elsewhere.

The Unite general secretary, Len McCluskey, said: "The government's refusal to address the growing scandal of zero-hours contracts is creating a sub-class of insecure and low-paid employment."

The Trades Union Congress will debate the issue on Sunday as unions pile pressure on the government for change. Vince Cable, the business secretary, is undertaking a review of zero-hours contracts for the government which is due by the end of this year. He has said that new legislation could be introduced but his inquiry has already been branded inadequate by Labour.

Cable said: "For some, these can be the right sort of employment contract, giving workers a choice of working patterns. However, for a contract that is now more widely used, we know relatively little about its effect on employers and employees." He said he would follow up "anecdotal evidence" of abuse, including in the public sector, of vulnerable workers "at the margins of the labour market" because it was important that workers were "fairly treated".

According to the Unite study, 36% of those on the contracts said they did not receive holiday pay and 77% got no sick pay. In a resounding retort to those who suggest workers can enjoy the benefits of flexible working, 87% said they would prefer not to be on a contract which could promise less than three hours work a week. "Employers are exploiting zero-hours contracts to dodge holiday and sick pay. This latest mutation of precarious working is leaving workers in limbo, unable to plan how much they will earn or when they will work from one week to next," McCluskey said.

The study found that, in geographical terms, those in the north-west of England were most affected – 78% – while the contracts were most likely to be signed by young people. Half of those under 30 said their work deal committed to employ them for no more than three hours a week. In terms of industry, agricultural workers were the most likely to be on zero-hours contracts, with all those who responded saying they had such terms imposed. The National Farmers Union, which represents many of those employing such workers, said zero-hours contracts were "essential" in the agriculture industry, particularly horticulture, as they gave farmers and growers the flexibility to deal with the inherent variations in crop ripening.

The NFU senior legal adviser James Potter said agricultural employers treated their zero-hours staff well: "The NFU believes that legislative reform is not necessary. Farmers and growers understand that workers expect reasonable weekly minimum hours and will vote with their feet if their expectations are disappointed."

Unite called on the government to restore collective bargaining for different industry sectors to stop the "race to the bottom" in "vulnerable" areas such as social care, hospitality, retail, food and logistics. The independent study by Mass 1, a research company, is the biggest to date, and encompasses a broad range of business sectors, from agriculture to transport and oil refining.

Its findings come after the Office for National Statistics revised up its estimate of the number of British workers on the flexible contracts by 25% to 250,000 last month, after being deluged with inquiries following a Guardian investigation into the issue. That investigation revealed that 90% of Sports Direct staff were employed on zero-hours contracts. Other firms using the controversial arrangements include Cineworld, McDonalds and even Buckingham Palace.

The revised ONS estimate was called into question only days later when a survey of 1,000 employers by the conducted by the Chartered Institute of Personnel and Development suggested there could be around 1 million workers in the UK on zero-hours contracts.