UNIONS have been pressing Labour to commit to ban zero-hours contracts, which have been increasingly controversial over the past few years.

The latest official figures from the Office for National Statistics showed that just under 700,000 workers were employed on zero-hours at the end of last year, while the number of contracts that did not guarantee a minimum number of hours was 1.8 million.

Paul Kenny, general secretary of the GMB union, said: "At long last the damage of zero-hours contracts is to be addressed. This news will be welcomed by the tens of thousands of people for whom the world of work is a daily lottery.

"Hopefully David Cameron will find himself on a zero-hours contract come May 8."

A study by Citizens Advice yesterday showed that workers on zero-hours contracts and irregular hours were afraid to speak out about their rights for fear of losing their job.

Research revealed that people contacting a bureau for help were more likely to have fluctuating hours or shift patterns than a year ago.

A survey of more than 300 staff at 100 CAB offices in England and Wales showed that irregular hours were as much of an issue as zero-hours contracts.

Most of the staff surveyed said people with fluctuating work have problems with debt or childcare and face delays to benefits.

Seven out of 10 staff said they were aware of cases where someone's hours or shifts worsened after they turned down work, or took holiday or sick leave.

"Budgeting, cutting down on fuel costs and being able to pay day-to-day bills is very difficult when you don't know how much work you will get," said the report.

"Not being able to show steady income or guaranteed future income can also be a barrier for people trying to get a mortgage or even privately rent a home."

Last year more than 220,000 people contacted Citizens Advice seeking help with employment problems.

Gillian Guy, chief executive of Citizens Advice, said: "We need flexibility in the labour market but not at the cost of fairness. The combination of low pay and unpredictable work patterns leaves too many people facing an uphill struggle to make ends meet from one month to the next.

"Zero-hours contracts have hit the headlines but many more workers are struggling at the sharp end of insecure jobs and unscrupulous employers.

"The next government needs to consider how to ease some of the downsides of flexible work patterns and make sure people in second-choice jobs don't fall prey to in-work poverty."

TUC general secretary Frances O'Grady said: "Exploitative zero-hours contracts are a gift for bad employers who can effectively hire and fire staff at will.

"Zero-hours contracts shift almost all power from the worker and give it to their boss. Anyone on such a contract has no guarantee of any work from one day to another. Put a foot wrong, and you can find yourself with little or no work."

The Prime Minister was pressed about the contracts during his TV interview last week, saying only one in 50 jobs were zero-hours contracts, and insisting they suited some people, including students who wanted flexibility.

Sports Direct chairman Keith Hellawell told MPs last week that 4,300 of the firm's 19,000 staff were permanent, with the rest employed as casuals, on zero-hours contracts.

They were told on a Thursday how many hours they would work the following week, he said.

The average number of hours worked for someone on a zero-hours contract is 25, with a third wanting more hours, studies have found.

Institute of Directors head of communications Christian May said: "Labour's proposals go too far. They are unnecessary and potentially damaging. Frankly, this is an example of politics trumping good policy.

"A cross-party consensus has already emerged that would ban the use of exclusivity clauses, but limiting the use of a zero-hours contract to just 12 weeks would apply rigid controls on an important element of our flexible labour market.

"When it comes to zero-hours contracts, the rhetoric simply does not match the reality. They are used by a little over 2% of workers, which can hardly be described as an epidemic. Nobody supports the misuse of these contracts, but demonising and ultimately outlawing them will simply risk jobs and undermine a labour market that has made us the envy of Europe."

Ukip economics spokesman Patrick O'Flynn said: "The spread of zero-hours contracts is yet another symptom of the over-supply of labour for working-class jobs because of open-door immigration from the EU.

"Big corporate employers have been able to bid down both pay and conditions because they know that workers can be found from the low-wage economies of southern and eastern Europe who will accept inferior terms."

University and College Union general secretary Sally Hunt said: "People on zero-hours contracts are unable to plan their lives on a month-by-month or even week-by-week basis. We welcome the proposal from Ed Miliband and would work with any party committed to seriously tackling the problem of casualisation in education.

"The flexibility of these types of contracts is not a two-way street and people who want security and a proper contract should be able to secure one. The lack of job security in our colleges and universities is a huge problem and the scale of insecure contracts would probably shock many students and parents."

The director of free-market think-tank the Adam Smith Institute, Eamonn Butler, said: "Forcing firms to give employees a regular contract after 12 weeks is effectively abolishing zero-hours contracts for 90% of the 1.8 million on them, and it would harm the very people it is intended to help.

"The UK's economic success is founded on labour market flexibility, and politicians need to be very careful before messing with it."