Workers on zero hours contracts are often afraid to look for other jobs and feel excluded from the sense of security given to full-time employees, according to a new study.
The conciliation service Acas said it was receiving around 70 calls a week about zero hours contracts, with a feeling of "effective exclusivity" of being tied to one employer emerging as a major concern.
Acas chairman Sir Brendan Barber, said: "Our analysis reveals that many workers on zero hours contracts experience a deep sense of unfairness and mistrust that go beyond the use of exclusivity clauses.
"A lot of workers on zero hours contracts are afraid of looking for work elsewhere, turning down hours, or questioning their employment rights in case their work is withdrawn or reduced.
"This deep rooted 'effective exclusivity' can be very damaging to trust and to the employment relationship. "There also appeared to be a lack of transparency on the terms of their contractual arrangements.
"Many people did not seem to even know that they were on a zero hours contracts and some believed they were on a permanent contract due to the length of their service."
The Government has been consulting on the use of zero hours contracts amid calls from unions and campaign groups to have them banned.
Labour has pledged to tackle abuses of zero hours contracts if it wins the next general election.
Acas said zero hours contracts with exclusivity clauses where there was no guarantee of work were likely to have a negative impact on employment relations.
Sir Brendan added: "These clauses undermine trust in the employment relationship and create insecurities for employees both in terms of their pay and their choices for working elsewhere."
Business Secretary Vince Cable said: "Acas's research supports what we have found in our review.
"Whilst zero hours contracts work for some, the use of exclusivity clauses and the lack of clear information can leave employees feeling vulnerable.
"That is why I launched a consultation last autumn which looked into the issues of transparency and exclusivity in the use of zero hours contracts, to ensure people are getting a fair deal.
"I want to make sure those looking to work flexibly under these types of contracts understand their rights and are not prevented from topping up their income by being tied exclusively to one employer. We will publish our response to the consultation shortly"