The number of workers on zero-hours contracts with no guarantee of work could be one million, four times higher than official estimates, according to new figures revealed on Monday.

A survey of 1,000 employers showed that around one in five employed at least one person on a zero-hours contract, the Chartered Institute of Personnel and Development (CIPD) said.

The data suggested that between three and four percent of all workers covered by the survey were on the contracts -- roughly one million workers across the UK.

The findings said employers in the voluntary and public sectors were more likely to use zero-hours contracts, along with those in hospitality.

Under the contract, employees agree to work as and when required. Critics say the contracts are abused and favour employers.

But a separate survey of contract employees found only 14 percent of workers said their employers failed to offer them sufficient work, according to the figures from CIPD's 2013 Labour Market Outlook.

The Office for National Statistics last week said 250,000 people in the country were on zero-hours contracts at the end of last year.

Unions believe these figures under-represent the reality and have criticised zero-hours contracts, saying most workers have no choice but to accept them.

"The vast majority of workers are only on these contracts because they have no choice. They may give flexibility to a few, but the balance of power favours the employers and makes it hard for workers to complain," said Dave Prentis, general secretary of the Unison union, calling for a ban on the contracts.

But CIPD chief executive Peter Cheese said the contracts could mean flexibility for employees and called for a closer look at them.

"The assumption that all zero-hours contracts are bad and the suggestion from some quarters that they should be banned should be questioned," Cheese added, while warning that employers must not use such contracts to avoid their responsibilities.