A Labour government would curb "exploitative" zero-hours contracts with a new right for workers to be given regular hours, Ed Miliband will announce today.

The Labour leader, speaking in Yorkshire, will explain that the entitlement will apply to employees after just 12 weeks of work.

"The problem of zero-hours contracts is at the heart of the key question in this election: who does our country work for? Does it work just for the rich and the powerful? Or does it work for working people - the people looking for a job, trying to find enough money to support a family, to make ends meet?" Miliband will argue.

He will add: "These zero-hours contracts have become a symbol of the Tories' failing economy with stagnant wages and falling productivity leaving a recovery which isn't reaching your front door and a deficit still at Downing Street's door."

The Office for National Statistics (ONS) found that there were 1.8 million of the controversial employment agreements in use by UK businesses in August last year, up from 1.4 million in January.

But the research body warned that the figures may not be comparable because of seasonal variants.

Zero-hours contracts have become a hot political subject in the UK and the government have outlawed exclusivity clauses, which prevented employees from working for more than one firm.

However, business bodies have argued that the agreements offer much needed flexibility for employers and help boost employment.

A survey from the Chartered Institute of Personnel and Development (Cipd) even found that zero-hours contract workers were on average more satisfied with their work-life balance than an average employee signed up to a regular agreement (65% vs 58%).

The Confederation of British Industry (CBI), which represents 190,000 British firms, warned Miliband not to "demonise" the controversial contracts.

"The UK's flexible jobs market has given us an employment rate that is the envy of other countries, so proposals to limit flexible contracts to 12 weeks are wide of the mark," said John Cridland, the director-general of the CBI.

"Of course action should be taken to tackle abuses, but demonising flexible contracts is playing with the jobs that many firms and many workers value and need.

"These proposals run the risk of a return to day-to-day hiring in parts of the economy, with lower stability for workers and fewer opportunities for people to break out of low pay."

Miliband's pledge will come with 36 days to go before the general election on 7 May, with Labour neck-and-neck (35% vs 35%) in the latest opinion poll from YouGov.